Description of St. Johns River Main-Stem Sites

All sites on the main stem of the St. Johns River have substantive tidal influence, and therefore, the calculated discharge is tidally filtered. Only one change, the addition of salinity calculation at St. Johns River at Buffalo Bluff near Satsuma, was made to the monitoring network during this year of the study.

The farthest upstream discharge monitoring site on the St. Johns River is in Astor, Fla., at the State Road 40 Bridge (table 1; river mile 127). Approximately 37 mi downstream, discharge is also calculated for the St. Johns River at Buffalo Bluff (river mile 90). Tidally filtered discharge values have been computed since 1994 at both Astor and Buffalo Bluff. Water temperature and specific conductance are also measured at Buffalo Bluff at an approximate depth of 6 ft below the average high tide level. These parameters have been measured since the 2017 water year, so salinity was calculated using that data for comparison in this study. Another water-quality monitoring station along the St. Johns River is located downstream at Dancy Point (river mile 71), where water temperature and specific conductance are measured at an approximate depth of 11.5 ft below the average high-tide level. Stage, water temperature, and specific conductance are monitored farther downstream near the right bank of the St. Johns River at Racy Point (river mile 64). The water-quality parameters are measured at an approximate depth of 2.5 ft below the average high-tide level. Water temperature and specific conductance are collected near the channel of the St. Johns River at Shands Bridge (river mile 50). This location was historically monitored from April 1995 to September 2001 at multiple depths and from April 2008 to September 2009 at the middle depth. The meter was reinstalled in May 2018 at the middle-depth location of 10 ft below the average high-tide level. Just downstream near the right bank, stage, water temperature, and specific conductance are measured at an approximate depth of 8.5 ft below the average high-tide level at the St. Johns River below Shands Bridge (river mile 49).

A monitoring station is located near the left bank of the St. Johns River just upstream from Buckman Bridge. Water temperature and specific conductance are measured at an approximate depth of 4 ft below the average high-tide level at this station, St. Johns River above Buckman Bridge (river mile 35). A monitoring station at Buckman Bridge (river mile 34) near the main channel measures stage, water temperature, and specific conductance. The water-quality parameters are measured with separate pump setups and intakes at two different depths to measure possible stratification. One water-quality meter is used in the chamber and monitors both water properties at each depth. All water-quality parameters are monitored at approximate depths of 8 ft (top) and 16 ft (bottom) below the average high-tide level. Water-quality monitoring stations at Christopher Point (river mile 29) and Marco Lake (river mile 25) measure water temperature and specific conductance at an approximate depth of 4 ft below the average high-tide level for each location. These sites are characterized by relatively shallow water having an average depth of less than 6 ft and represent river conditions in seagrass beds near the shore.

The St. Johns River at Jacksonville, Fla., data collection station, located near downtown Jacksonville at Acosta Bridge (river mile 23), is the farthest downstream gage that calculates discharge. Stage and discharge were previously monitored before the dredging study began, but water temperature and specific conductance data collection began in October 2015 at an approximate depth of 11 ft below the average high-tide level. The monitoring equipment was moved to the center of the channel in April 2018 because of fender construction, and the intake was installed at a new depth of 17 ft below the average high-tide level. Velocity and streamflow direction data collection were added in October 2017. Tidally filtered discharge values have been computed since 1996. A monitoring station at Dames Point Bridge in Jacksonville (river mile 10) measures stage, velocity, water temperature, and specific conductance; calculation of salinity began in October 2016. All water-quality data are collected at approximate depths of 15 ft (top) and 22 ft (bottom) below the average high-tide level by using one water-quality meter and pump setup with separate intakes, similar to those used at the station at Buckman Bridge to measure possible stratification. At Dames Point, the collection of stage data began in October 2017, and the collection of velocity and streamflow direction data began in May 2018.

Description of Tributary Sites

No changes to the tributary monitoring network occurred during this year of the study. A monitoring station on Julington Creek at Old St. Augustine Road calculates discharge 6.7 mi upstream from the confluence of Julington Creek and the St. Johns River (table 1). A nearby water-quality monitoring station measures water temperature and specific conductance at an approximate depth of 3.5 ft below the average high-tide level and is located approximately 3 mi downstream from Old St. Augustine Road on Julington Creek at Hood Landing.

Durbin Creek is monitored for stage, discharge, water temperature, specific conductance, and salinity 3.5 mi upstream from the confluence with Julington Creek and 6.8 mi upstream from the confluence with the St. Johns River. The water-quality parameters are measured at an approximate depth of 3 ft below the average high-tide level. These locations are all tidally influenced, but large amounts of rainfall, resulting in increased discharge, can obscure the tidal signal.

Ortega River is monitored for stage and discharge at Argyle Forest Boulevard (Kirwin Road), 11 mi upstream from the confluence of the Ortega and St. Johns Rivers. Discharge has been computed intermittently at this location since 2002, and a stage-discharge rating was used for discharge computation during the study period, as no tidal influence is apparent in the stage data. A water-quality monitoring station that measures water temperature and specific conductance at an approximate depth of 6.5 ft below the average high-tide level is located at the bridge on Collins Road approximately 3 mi downstream from Kirwin Road at the Ortega River. Tidal influence is evident in the salinity data during times of low flow or increased stage in the St. Johns River.

The monitoring station on Cedar River measures stage, water temperature, and specific conductance at San Juan Avenue, 1.5 mi upstream from the confluence of the Cedar and St. Johns Rivers. The water-quality parameters are monitored at an approximate depth of 6 ft below the average high-tide level. Historical, tidally filtered discharge values have been computed intermittently since 2002. Wind, tide, and rainfall all substantively affect flow and salinity at Cedar River, and vertically stratified, bidirectional flow is commonly measured when strong winds occur opposite the direction of flow.

The station at Pottsburg Creek near South Jacksonville measures stage and is 7.3 mi upstream from the confluence of Pottsburg Creek and the St. Johns River. A stage-discharge rating was used for discharge computation during the study period. The data indicate tidal influence when stage is very low in the creek and elevated in the St. Johns River, but discharge measurements confirm the validity of the stage-discharge relation. Pottsburg Creek is monitored for stage, discharge, water temperature, specific conductance, and salinity at U.S. 90, 5.2 mi upstream from the confluence of Pottsburg Creek and the St. Johns River. The water-quality parameters are monitored at an approximate depth of 4 ft below the average high-tide level. This location has a more pronounced tidal signal than the upstream location on Pottsburg Creek, and discharge and salinity are affected primarily by rainfall and elevated stage in the river.

The monitoring station for Trout River calculates discharge at Lem Turner Road, 5 mi upstream from the confluence of the Trout and St. Johns Rivers. Water temperature and specific conductance are measured 9.3 mi upstream from the confluence at an approximate depth of 6 ft below the average high-tide level. Broward River is monitored for stage, discharge, water temperature, specific conductance, and salinity near Biscayne Boulevard, 6.3 mi upstream from its confluence with the St. Johns River. The water-quality parameters are monitored at an approximate depth of 4.5 ft below the average high-tide level. Discharge and salinity at these stations have a pronounced tidal signal, even when stage is elevated.

Dunn Creek is monitored for stage, discharge, water temperature, specific conductance, and salinity at Dunn Creek Road, 5.3 mi upstream from the confluence of Dunn Creek and the St. Johns River. The monitoring station for Clapboard Creek calculates discharge near Sheffield Road, 4.5 mi upstream from the confluence of Clapboard Creek and the St. Johns River. Water temperature and specific conductance are measured at a location 0.5 mi upstream. The water-quality parameters are monitored at approximate depths of 5.5 and 6.5 ft below the average high-tide level. Discharge and salinity fluctuations at these stations are dependent on rainfall in the relatively small drainage area and on wind effects from the St. Johns River, owing to the proximity of the sites to the river and Atlantic Ocean.

Rainfall

Duval County rainfall for the 2020 water year (October 2019–September 2020) was 5.6 inches (in.) above the long-term average (SJRWMD, 2021). Monthly total rainfall ranged from 2.5 in. below average in January to 3.8 in. above average in June (fig. 3). Duval County had the second-highest annual rainfall compared to the cumulative long-term average of the five counties included in this study (SJRWMD, 2021). Annual rainfall averaged 1.3 in. above and 3.4 in. below average for Clay and St. Johns Counties, respectively, and annual rainfall was less for both counties than for Duval County (SJRWMD, 2021). The highest monthly rainfall total for Clay County occurred in June and was 2.7 in. above average, and the highest positive monthly departure for St. Johns County, 1.9 in. above average, occurred in December. The lowest monthly rainfall totals for the two counties occurred in March and were 2.5 below average for Clay County and 3.2 in. below average for St. Johns County (figs. 4 and 5). Of the five counties compared for this study, the two northernmost counties, Duval County and Clay County, ranked second and third for rainfall totals in the 2020 water year.

Rainfall totals for Putnam County and Volusia County were 3.4 in. below and 9.1 in. above their average yearly totals, respectively (SJRWMD, 2021). Rainfall for Putnam County ranged from 2.0 in. above average in December to 2.5 in. below average in March (fig. 6). Rainfall for Volusia County ranged from 4.3 in. above average in October to 3.0 in. below average in March (fig. 7). Rainfall for each of the five counties was below the average monthly total from January to March except Duval County, which was slightly above average in February.

Main-Stem Sites

The parameters collected on the main stem of the St. Johns River are affected by many factors, including tidal influence, rainfall, and wind. Heavy rainfall typically increases discharge and decreases salinity, and the tidal influence introduces more fluctuations than a nontidal system. Strong easterly winds can increase salinity in the St. Johns River as ocean water with higher salinity is pushed upstream and can lower or even produce negative discharge for multiple days. When the wind calms or changes direction, discharge typically increases sharply as this ocean water and other tributary inflows exit the river.

St. Johns River at Astor, Florida—Daily tidally filtered discharge at Astor ranged from −3,720 to 9,800 cubic feet per second (ft³/s) during the 2020 water year, with an annual mean of 2,900 ft³/s (fig. 8). Tidally filtered discharge was estimated for 5 days in December, 12 days in June, and 5 days in September. A comparison of historical annual mean tidally filtered flows indicated that 2020 streamflow was at the 56th percentile of the 26 years of record; the median tidally filtered annual mean flow for the period of record is 2,820 ft³/s (fig. 9). The annual peak discharge occurred in September after 3 consecutive months of above-average rainfall in Volusia County (fig. 7).

St. Johns River at Buffalo Bluff near Satsuma, Florida—Daily tidally filtered discharge at Buffalo Bluff ranged from −14,900 to 18,100 ft³/s during the 2020 water year, with an annual mean of 4,780 ft³/s (fig. 10). Salinity ranged from 0.3 to 0.7 parts per thousand (ppt) over this same period, with a median and mean of 0.5 ppt (fig. 11). There were 35 days of missing salinity data from June to July and 5 days of missing discharge data in August caused by equipment malfunction. A comparison of historical annual mean tidally filtered flows indicated that 2020 streamflow was at the 65th percentile of the 27 years of record; the median tidally filtered flow for the period of record is 4,480 ft³/s (fig. 12). The lowest tidally filtered discharge for the 2020 water year occurred in late February during a period of below-average rainfall in Putnam County (fig. 6). The annual mean and median salinity are within 0.2 ppt for the 4 years salinity was calculated in the study (fig. 13).

St. Johns River at Dancy Point near Spuds, Florida—Salinity on the St. Johns River at Dancy Point ranged from 0.3 to 0.6 ppt during the 2020 water year, with a median and mean of 0.5 ppt (fig. 14). The salinity data show fluctuations caused by tidal effects and rainfall, and a gradual increase from March to May. The annual mean and median salinity are within 0.2 ppt for the 4 years salinity was calculated in the study (fig. 15).

St. Johns River at Racy Point near Hastings, Florida—Salinity on the St. Johns River at Racy Point ranged from 0.3 to 0.6 ppt during the 2020 water year, with a median of 0.4 ppt and mean of 0.5 ppt (fig. 16). There were 4 days of missing salinity data each in April and May, 14 days in June, and 5 days in July caused by equipment malfunction. Like computed salinity at the Buffalo Bluff site, computed salinity at Racy Point gradually increased from March to May. The 2020 mean salinity was the highest by 0.1 ppt over the 3 years considered for this study (fig. 17).

St. Johns River Shands Bridge near Green Cove Springs, Florida—Salinity on the St. Johns River Shands Bridge ranged from 0.3 to 2.8 ppt during the 2020 water year, with a median and mean of 0.5 ppt (fig. 18). There were 44 days of missing salinity data from May to June and 3 days of missing salinity data in September, all caused by equipment malfunction. Salinity peaked in early April after below-average rainfall occurred in March in St. Johns County (fig. 5). A boxplot of salinity data shows a higher median, mean, and maximum for 2020 compared to 2018 and 2019 (fig. 19).

St. Johns River below Shands Bridge near Green Cove Springs, Florida—Salinity on the St. Johns River below Shands Bridge ranged from 0.3 to 4.5 ppt during the 2020 water year, with a median of 0.5 ppt and mean of 0.8 ppt (fig. 20). Daily salinity data were not computed for many days from October to January and for 10 days from June to July because of equipment malfunction. The maximum salinity of 4.5 ppt is higher than the maximum of 2.8 ppt at the nearby station at Shands Bridge, most likely because the monitoring equipment is located near the right bank instead of near the center of the river channel. In addition, salinity was not computed at the Shands Bridge site on the day the peak occurred below Shands Bridge. The 2020 median salinity is the highest among those computed for the 4 years of the study, and the 2020 mean salinity ranked second (fig. 21).

St. Johns River above Buckman Bridge at Jacksonville, Florida—Salinity on the St. Johns River above Buckman Bridge ranged from 0.4 to 12 ppt during the 2020 water year, with a median of 1.5 ppt and mean of 2.6 ppt (fig. 22). Daily maximum salinity peaked above 10 ppt in February, April, and May before above-average rainfall in Clay County in June lowered the salinity (fig. 4). The median and mean salinity for 2020 were both the highest among those computed for the 3 years of the study, although 2018 has incomplete record because the monitoring station was installed that year (fig. 23).

St. Johns River Buckman Bridge at Jacksonville, Florida—Salinity on the St. Johns River at Buckman Bridge ranged from 0.3 to 16 ppt for the top location during the 2020 water year, with a median of 1.6 ppt and mean of 2.9 ppt, and from 0.3 to 18 ppt for the bottom location, with a median of 1.8 ppt and mean of 3.2 ppt (figs. 24 and 25). The monitoring station did not record water-quality data at either location for 28 days from February to March and 6 days in May because of equipment malfunction. Salinity during the year was generally lower at the top location relative to the bottom. Salinity at both locations increased gradually from March through May before higher rainfall in June in Duval and Clay Counties lowered the salinity (figs. 3 and 4). The 2020 median and mean salinity at both the top and bottom locations ranked second among those computed for the 4 years of the study (figs. 26 and 27).

St. Johns River at Christopher Point near Jacksonville, Florida—Salinity at Christopher Point ranged from 0.4 to 24 ppt during the 2020 water year, with a median of 2.9 ppt and mean of 4.2 ppt (fig. 28). The annual maximum salinity occurred in February after very low rainfall in Duval County in January (fig. 3). The 2020 maximum salinity ranked second among those computed for the 5 years of the study, and the median and mean salinity both ranked third (fig. 29).

St. Johns River below Marco Lake at Jacksonville, Florida—Salinity at Marco Lake ranged from 0.4 to 28 ppt during the 2020 water year, with a median of 6.2 ppt and mean of 7.0 ppt (fig. 30). Salinity was not computed for 6 days in July because of equipment malfunction. As at the Christopher Point site, the annual maximum salinity at this site occurred in February after very low rainfall in Duval County in January (fig. 3). The 2020 median salinity ranked third among those computed for the 5 years of the study, and the mean and maximum salinity ranked second (fig. 31).

St. Johns River at Jacksonville, Florida—Daily tidally filtered discharge at the St. Johns River at Jacksonville site ranged from −39,900 to 38,300 ft³/s during the 2020 water year, with an annual mean of 5,830 ft³/s (fig. 32). Salinity ranged from 0.4 to 28 ppt over this same period, with a median of 7.6 ppt and mean of 8.4 ppt (fig. 33). Salinity exceeded 25 ppt three times during the water year in January, February, and May. Salinity was not computed for 3 days in February because of equipment malfunction. A comparison of historical annual mean tidally filtered flows indicated that the 2020 streamflow was at the 52d percentile of the 24 years of record; the median tidally filtered flow for the period of record is 5,660 ft³/s (fig. 34). Annual mean tidally filtered streamflow record was used from 1997 onward because of data collection gaps in the historical data prior to 1997. The 2020 mean and median salinity ranked the highest among those computed for the 5 years of the study, but the maximum salinity ranked fourth (fig. 35).

St. Johns River Dames Point Bridge at Jacksonville, Florida—Salinity on the St. Johns River at Dames Point Bridge ranged from 3.7 to 34 ppt for the top location during the 2020 water year, with a median of 23 ppt and mean of 22 ppt, and from 4.4 to 34 ppt for the bottom location, with a median of 24 ppt and mean of 23 ppt (figs. 36 and 37). Salinity was not calculated for 28 days in February and March at both locations and for 6 days at the top location in September because of equipment malfunction. Salinity during the year was generally lower at the top location relative to the bottom and fluctuated weekly at both locations because of upstream conditions. The 2020 mean and median salinity at the top location ranked the highest among those calculated for the 4 years of the study, but the mean tied that of 2017 (fig. 38). For the bottom location, the 2020 median salinity ranked the highest among those calculated for the 4 years of the study, and the mean ranked second (fig. 39).

Tributary Sites

The parameters collected on the tributaries of the St. Johns River are mainly affected by rainfall and, at most sites, by tidal fluctuations. Heavy rainfall typically increases discharge and decreases salinity, and the tidal influence introduces more daily fluctuation at tidal sites than at nontidal sites. Like the main stem, tributaries of the St. Johns River are also affected by strong easterly winds. Because of their distance from the ocean, tributary sites and main-stem sites farther upstream require prolonged or extremely strong winds, such as those from a hurricane, to experience increased salinity.

Julington Creek at Old St. Augustine Road near Bayard, Florida—Daily tidally filtered discharge at Julington Creek ranged from −30.7 to 368 ft³/s during the 2020 water year, with an annual mean flow of 28.7 ft³/s (fig. 40). The highest discharge peak in 2020 occurred in June as a result of above-average rainfall in Duval County (fig. 3). Discharge was not calculated for 5 days in July because of equipment malfunction.

Julington Creek at Hood Landing near Bayard, Florida—Salinity at Julington Creek at Hood Landing ranged from 0.1 to 3.5 ppt during the 2020 water year, with a median of 0.3 ppt and mean of 0.6 ppt (fig. 41). The daily maximum salinity exceeded 1.0 ppt during low flows from March to June and decreased after the discharge peak in June (fig. 40). The 2020 median salinity ranked the highest among those computed for the 5 years of the study, and the mean salinity ranked second (fig. 42).

Durbin Creek near Fruit Cove, Florida—Daily tidally filtered discharge at Durbin Creek ranged from −39.5 to 270 ft³/s during the 2020 water year, with an annual mean of 50.8 ft³/s (fig. 43). The highest discharge peak in 2020 occurred in September as a result of above-average rainfall in St. Johns County (fig. 5). Salinity ranged from 0.1 to 1.5 ppt, with a median and mean of 0.1 ppt (fig. 44). The boxplot for Durbin Creek salinity data in 2020 shows the same mean and median for all 5 years of the study; the maximum salinity ranked second among those computed for the 5 years of the study (fig. 45). The boxplot includes no whiskers because of the low and steady salinities of freshwater flows through the creek.

Ortega River at Kirwin Road near Jacksonville, Florida—Daily discharge for Ortega River ranged from 3.53 to 1,180 ft³/s during the 2020 water year, with an annual mean of 58.1 ft³/s (fig. 46). Like Julington Creek, Ortega River had the largest discharge peak in June. The peak streamflow for the period of record ranked in the 73d percentile of the 13 years of record (fig. 47). The peak streamflow record does not include 2016, which has incomplete record because the monitoring station was installed that year.

Ortega River salinity at Jacksonville, Florida—Salinity at Ortega River ranged from 0.0 to 13 ppt during the 2020 water year, with a median of 0.1 ppt and mean of 0.6 ppt (fig. 48). Peak maximum daily salinity values were calculated for January to May during periods of discharge below 100 ft³/s in Ortega River (fig. 46). The 2020 maximum and median salinity ranked the highest among those calculated for the 5 years of the study, and the mean salinity tied with 2019 for second (fig. 49).

Cedar River at San Juan Avenue at Jacksonville, Florida—Daily tidally filtered discharge at Cedar River ranged from −52.9 to 1,020 ft³/s during the 2020 water year, with an annual mean of 44.2 ft³/s (fig. 50). Salinity ranged from 0.0 to 15 ppt, with a median of 1.0 ppt and mean of 2.7 ppt (fig. 51). Daily tidally filtered discharge was not calculated for 7 days in December, 3 days in January, 4 days in March, 1 day in May, 4 days in June, 3 days in August, and was estimated for 4 days in June because of equipment malfunction. As at other tributary sites, the peak discharge at Cedar River occurred during a sharp rise in June. The 2020 mean salinity ranked second among those computed for the 5 years of the study, and the median salinity ranked third (fig. 52).

Pottsburg Creek near South Jacksonville, Florida—Daily discharge for Pottsburg Creek at Bowden Road ranged from 1.94 to 437 ft³/s during the 2020 water year, with an annual mean flow of 26.0 ft³/s (fig. 53). Daily mean discharge peaked above 200 ft³/s in October, June, and August, all of which had above-average rainfall in Duval County (fig. 3). The peak streamflow for the period of record ranked in the 45th percentile of the 10 years of record (fig. 54).

Pottsburg Creek at U.S. 90 near South Jacksonville, Florida—Daily tidally filtered discharge at Pottsburg Creek at U.S. 90 ranged from −11.4 to 488 ft³/s during the 2020 water year, with an annual mean flow of 46.0 ft³/s (fig. 55). Daily tidally filtered discharge was not calculated for 3 days in October, 10 days in November, 7 days in December, and 9 days in June because of equipment malfunction. The daily discharge peaked two times over 300 ft³/s, and the peak that occurred in June was the highest since September 2017. Salinity ranged from 0.0 to 12.0 ppt, with a median of 0.4 ppt and mean of 1.1 ppt (fig. 56). Peak salinity exceeded 10 ppt for 9 consecutive days in May during periods of below-average rainfall in Duval County (fig. 3). The maximum salinity tied 2016 and 2017 for highest among those computed for the 5 years of the study, and the 2020 mean salinity tied with 2016 for second (fig. 57).

Trout River near Jacksonville, Florida—Daily tidally filtered discharge at Trout River ranged from −320 to 1,190 ft³/s during the 2020 water year, with an annual mean flow of 35.5 ft³/s (fig. 58). Daily tidally filtered discharge was not calculated for 5 days in October and 9 days in June because of equipment malfunction. The daily discharge exceeded 600 ft³/s two times during the period, and the peak that occurred in August was the highest since September 2017.

Trout River below U.S. 1 at Dinsmore, Florida—Salinity at Trout River below U.S. 1 ranged from 0.0 to 14 ppt during the 2020 water year, with a median of 0.5 ppt and mean of 2.3 ppt (fig. 59). Daily maximum salinity values peaked above 10 ppt during portions of April through June. The second-highest maximum yearly salinity of the 5-year period of record (14 ppt) occurred on multiple days in late May and early June and ended when flow increased in Trout River (fig. 58). The 2020 mean and median salinity ranked fourth among those calculated for the 5 years of the study (fig. 60).

Broward River below Biscayne Blvd. near Jacksonville, Florida—Daily tidally filtered discharge at Broward River ranged from −46.5 to 398 ft³/s during the 2020 water year, with an annual mean flow of 25.6 ft³/s (fig. 61). Discharge was relatively consistent during the period, with 7 peaks of daily tidally filtered discharge exceeding 100 ft³/s. Salinity ranged from 0.0 to 14 ppt, with a median of 0.6 ppt and mean of 1.7 ppt (fig. 62). Salinity remained below 6 ppt most of the year but exceeded 10 ppt in October, November, and April (fig. 62). The 2020 median, mean, and maximum salinity ranked third among those calculated for the 5 years of the study (fig. 63).

Dunn Creek at Dunn Creek Road near Eastport, Florida—Daily tidally filtered discharge at Dunn Creek ranged from −5.62 to 172 ft³/s during the 2020 water year, with an annual mean flow of 25.8 ft³/s (fig. 64). Daily tidally filtered discharge was estimated for 5 days in December because of equipment malfunction. The cumulative 4-month rainfall in Duval County was 3.62 in. above the long-term average from June to September, and the daily tidally filtered discharge peaked over 100 ft³/s five times in that period (fig. 3). Salinity during the 2020 water year ranged from 0.0 to 17 ppt, with a median of 1.9 ppt and mean of 3.1 ppt (fig. 65). Daily maximum salinity exceeded 15 ppt for 8 days from April to May and remained below 7 ppt from mid-June to mid-September. The 2020 maximum and mean salinity tied with 2016 for second among those calculated for the 5 years of the study, and the median salinity ranked third (fig. 66). Salinity was not calculated for 2 days in April because of equipment malfunction.

Clapboard Creek near Jacksonville, Florida—Daily tidally filtered discharge at Clapboard Creek ranged from −74.1 to 278 ft³/s during the 2020 water year, with an annual mean flow of 61.3 ft³/s (fig. 67). Tidally filtered discharge remained below 200 ft³/s for the entire period except during two peaks in October and two in September. The highest peak flow in September ranked second, and the annual mean flow ranked first for the 5-year period of record considered for this study.

Clapboard Creek above Buckhorn Bluff near Jacksonville, Florida—Salinity at Clapboard Creek ranged from 1.0 to 32 ppt during the 2020 water year, with a median of 21 ppt and mean of 20 ppt (fig. 68). The lowest salinity occurred in September when discharge increased through Clapboard Creek (fig. 67). The 2020 mean salinity ranked fourth among those calculated for the 5 years of the study, and the median salinity tied with 2017 and 2019 for second (fig. 69).

Discharge and Salinity Site Comparison

Analysis of the annual mean tidally filtered discharge for the 2020 water year along the main stem of the St. Johns River indicates that streamflow increased with distance downstream, as expected (fig. 70). Relative to the 2019 water year, annual mean discharge for the 2020 water year was higher for two of the main-stem sites and identical for the other site. Over the 5 years considered for this study, the second-highest annual mean discharge was computed at the St. Johns River at Astor site , the Buffalo Bluff site tied for second, and the Jacksonville site ranked third. Of the tributaries, annual mean discharge was highest at Clapboard Creek, followed by Ortega River, Durbin Creek, Pottsburg Creek at U.S. 90, Cedar River, Trout River, Julington Creek, Pottsburg Creek near South Jacksonville, Dunn Creek, and Broward River, whose annual mean was lowest (fig. 71). Annual mean discharge at 8 of the 10 tributary monitoring sites was higher for the 2020 water year than for the 2019 water year, and the computed annual mean flow at Clapboard Creek was the highest over the 5 years considered for this study (fig. 71). The countywide annual rainfall for the 2020 water year was above the average in three of the five counties, higher in three of the five counties for the 2020 water year relative to the 2019 water year, and ranked third among the annual rainfall totals for the past 5 years (fig. 72).

Annual mean salinity for the main-stem sites generally decreased with distance upstream, which was expected (fig. 73). Of all sites in 2020, Dames Point had the highest annual mean salinities, which were 22 ppt near the surface and 23 ppt near the bottom of the water column. The 2020 annual mean salinity was higher than that of 2019 at all main-stem sites (fig. 73). The 2020 salinity at the main-stem sites, when compared to all 5 years of the study, ranked first at Dancy Point, Racy Point, Shands Bridge, below Shands Bridge, above Buckman Bridge, and at Jacksonville (Acosta Bridge), tied for first at Buffalo Bluff, second at Buckman Bridge (top and bottom) and Dames Point (top and bottom), and third at Christopher Point and Marco Lake.

Clapboard Creek had the highest annual mean salinity (20 ppt) of all tributaries in 2020 because of its proximity to the Atlantic Ocean. Durbin Creek salinity (0.1 ppt) was the lowest of all monitoring locations and was slightly lower than those of Julington Creek and Ortega River (both 0.6 ppt) (fig. 74). A simple comparison of annual mean salinity of tributary sites between the 2019 and 2020 water years indicates salinity was higher in 2020 at all locations except Trout River, Dunn Creek, and Clapboard Creek, which were lower, and Durbin Creek, which remained the same (fig. 74). The 2020 salinity at the tributary sites, when compared to the preceding 4 years of the study, ranked second at Ortega River and Julington Creek, tied for second at Dunn Creek, ranked third at Cedar River, Pottsburg Creek, and Broward River, and ranked fourth at Trout River and Clapboard Creek. Durbin Creek salinity remained the same at 0.1 ppt for all 5 years. The annual rainfall total for Duval County ranked second among those computed for the 5 years considered for this study (fig. 72).