Scientific Investigations Report 2007–5186
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5186
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Water year 2000 was selected to compare the discharges of TN, TP, and SS in the Columbia River and Puget Sound Basins. The discharge was the annual mean load in WY 2000, expressed in units of pounds per day for TN and TP, and tons per day for SS. Annual mean load is defined as the average load for each day of one year. WY 2000 was selected because it represented a typical or average stream flow condition in the Pacific Northwest (see earlier section of report titled “Selection of Analysis Period”). The following three sections provide detailed descriptions of the load and transport of nutrients and sediment in the Columbia River, Willamette River, and Snake Rivers. Estimated annual main-stem, monitored tributary, and point-source loads for the three river systems are included in tables 8, 9, and 10. Although only the tributaries for which loads were calculated could be included in the analysis, the tables and the information shown in figures 11 through 21 provide some insight into the relative contribution of tributary and point source loads to rivers.
In WY 2000, the Snake, Yakima, Deschutes, and Willamette Rivers contributed most of the load discharged to the Columbia River, while point-source loads to the river (almost exclusively from municipal wastewater treatment plants) generally equaled a small percentage of the total in-stream nutrient load. Annual loads from point sources discharging directly to the Columbia River in WY 2000 represented about 6 percent and 9 percent, respectively, of the estimated annual mean TN and TP load near the mouth of the river at Beaver Army Terminal (site 44, fig. 1). These loads also were equal to 10 percent and 14 percent, respectively, of the combined annual mean TN and TP loads discharged from the Snake, Yakima, Deschutes, and Willamette Rivers. Annual WY 2000 TN and TP loads from the largest point sources discharging directly to the Columbia River, which are in the fourth quartile in figure 9 (defined as those with loads exceeding 75 percent of the loads from all the facilities) are shown in figure 10. The City of Portland’s WWTP is the largest point-source discharger in the Columbia Basin. The annual TN and TP loads from this facility, which discharges directly to the Columbia River instead of the Willamette River, equaled 2.3 percent and 2.9 percent, respectively, of the annual in-stream TN and TP load at Beaver Army Terminal (site 44, table 1) in water year 2000.
Annual point-source loads of TN and TP discharged directly to the Willamette River were 11 percent and 36 percent, respectively, of the annual mean load discharged from the Willamette to the Columbia River. Combining the nutrient load from the City of Portland’s WWTP with the point-source nutrient load to the Willamette River increased these contributions to the Columbia River to 18 and 42 percent, respectively. Because most of the City of Portland lies within the Willamette Basin, these percentages represent the Willamette Basin’s total contribution to the Columbia River in WY 2000. Annual point-source loads of TN and TP discharged directly to the Snake River were 2 and 12 percent, respectively, of the annual mean load discharged from the Snake to the Columbia River. Although the drainage area for the Snake River is about 9 times larger than that for the Willamette, the Snake River’s point-source contribution to the Columbia River was about one half of the Willamette River’s contribution. The predicted annual mean loads for each of the main-stem sites on the Columbia River, the Willamette River, and the Snake River, as well as for sites located near the mouths of the Puget Sound tributaries for WY 2000 are included in appendix A. Further details regarding these loads are described in the following sections. An interpretation of nutrient and sediment transport in relation to landscape characteristics can be found in the section titled “Nutrient and Suspended-Sediment Yields”.
Annual loads from point sources discharging directly to the Columbia River in WY 2000 represented about 6 percent and 9 percent, respectively, of the annual mean TN and TP load near the mouth of the Columbia River at Beaver Army Terminal (site 44, fig. 1). These point-source loads also were small compared to annual mean loads discharged from the Snake and Willamette Rivers to the Columbia River. Annual WY 2000 TN and TP loads from the largest point sources discharging directly to the Columbia River (those in the fourth quartile in figure 9, or with loads exceeding 75 percent of the loads from all the facilities) are shown in figure 10. The City of Portland’s WWTP is the largest point-source discharger in the basin. The annual TN and TP loads from this facility, which discharges directly to the Columbia River instead of the Willamette River, equaled 2.3 percent and 2.9 percent, respectively, of the annual in-stream TN and TP load at Beaver Army Terminal (site 44, table 1) in water year 2000.
Annual point-source loads of TN discharged directly to the Willamette River were 11 percent of the annual mean load discharged to the Columbia River from the Willamette River, and the annual point-source loads of TP were 36 percent of the annual mean load discharged to the Columbia River. Aggregating the nutrient load from the City of Portland’s WWTP with the point-source nutrient load to the Willamette River increased the contributions to 18 and 42 percent, respectively. These percentages represent the total contribution of the Willamette Basin to the Columbia River in WY 2000. The population density (based on 2000 census data) was six times greater in the Willamette Basin compared to the population density in the Columbia River Basin overall.
Annual point-source discharges of TN and TP made up 2 and 12 percent, respectively, of the annual mean load discharged from the Snake River to the Columbia River. Although the Snake River drainage area is greater than the Willamette River drainage area, the Snake River point-source contribution to the Columbia River was considerably less than that of the Willamette River.
Six water-quality monitoring stations were used to assess nutrient and sediment transport on the main stem of the Columbia River. Stations in Washington were at Northport (RM 735, fig. 1, site 86), Grand Coulee (RM 596; fig. 1, site 77), and Vernita Bridge near Priest Rapids Dam (RM 388, fig. 1, site 51). Stations in Oregon were at Warrendale (RM 189; fig. 1., site 38), river marker 47 upstream of the Willamette River (RM 103; fig. 1, site 40), and Beaver Army Terminal (RM 54; fig. 1, site 44). In addition, 11 water-quality stations located on or near the mouths of Columbia River tributaries and 35 point sources discharging directly to the Columbia River, were used to assess nutrient and sediment load to the main stem of the river (fig. 11).
In WY 2000, the Columbia River discharged an average of 570,000 lb/d of TN to the Pacific Ocean (on the basis of the annual mean load at Columbia River at Beaver Army Terminal, fig. 1, site 44). In addition, an average of nearly 55,000 lb/d of TP, and about 14,000 ton/d of SS were discharged at Beaver Army Terminal. Tributary inputs to the Columbia River downstream of this site were small and did not appreciably affect the estimated load (Fuhrer and others, 1996). Nutrient and suspended-sediment discharges from the Columbia River represent contributions to the main stem over a distance of 700 river miles, but permanent losses and temporary storage also occur during transport over this distance. Nitrogen and phosphorus, important nutrients in aquatic ecosystems, are removed by plants and other forms of aquatic life. Nitrogen also can be lost to the atmosphere as a gas (denitrification), and nitrogen and phosphorus can be incorporated into the streambed sediment as a solid through settling of algae and sediment particles. All these processes can affect the transport of nutrients through rivers and to estuaries. Therefore, the total quantity of nutrients delivered to the Columbia River is not conservative and may not necessarily exit through the Columbia estuary. Similarly, sediment (and sorbed nutrients) can become trapped behind main-stem dams and delivered to the surrounding land through irrigation diversions.
Water-quality stations along the main stem of the Columbia River were used to delineate five reaches for analysis (table 7). An attempt to account for the streamflow was made on each of these reaches by summing the monitored streamflow contributions to the reach by the tributaries and comparing that value to the accumulation of flow at the end of the reach (table 7). This exercise was not done to account for the gain and loss in streamflow within each reach: such an accounting would require information from all streamflow management actions and natural processes that divert water, return water, or otherwise consume water (including evaporative losses). Instead, the accounting was performed to provide some perspective on the tributary discharges of TN, TP, and SS for the monitored tributaries (those meeting the screening criteria for load estimation). This accounting provided a basic understanding of a tributary’s significance as a nutrient or suspended-sediment source. Although the sum of the measured tributary inflows for reach 1 was nearly equal to the gain within this reach during WY 2000, the total contribution from tributary inflows was actually greater than the gain if the diversion of irrigation water from the reach was included (annual mean of about 3,700 ft3/s for WY 2000 for the diversion at Grand Coulee Dam). In reach 2, the total contribution from tributary streamflow also was greater than the gain in streamflow over the reach. The results on the flow accounting for reaches 1 and 2 indicate that the Columbia River may have been losing water to the regional ground-water system between Northport and Grand Coulee. A load balance was determined to be appropriate for reaches 3 and 4 because these reaches had no monitored tributary inputs. A load balance for reach 5 also was appropriate, with some caution regarding SS. An earlier study of the lower Columbia River (Fuhrer and others, 1996) determined that, except for the Willamette River, the streamflow and nutrient load contributions from the tributaries in this reach generally were minor (less than 5 percent in aggregate of the main-stem values). The one exception was the SS load from the Cowlitz River (the second largest tributary in the reach), which was about 22 percent of the main-stem SS load and was comparable to the load from the Willamette River. Because the Cowlitz River did not meet the screening criteria for load estimation, contributions of SS from this tributary were not considered in the load balance.
Discharge of TN and TP increased in reach 1, a distance of 139 river miles from Northport to Grand Coulee (fig. 12 and 13, and table 8). In this reach, the discharge of TN from the Spokane River accounted for nearly two-thirds of the gain in the annual mean load of TN (fig. 12) and slightly less than one-third of the gain in the annual load of TP (fig. 13). Point-source contributions of TN and TP in this reach, however, were negligible. The SS discharged at Northport decreased by more than 40 percent in reach 1 to downstream of Grand Coulee, likely the result of sediment deposition in the backwater impoundment behind Grand Coulee (Lake Roosevelt) and irrigation diversions (table 8 and fig. 14). The discharges in reach 2 increased for TN and SS, and decreased for TP (figs. 12, 13, and 14). Point-source discharges of TN and TP to the main stem in this reach were negligible.
Several large tributaries join the Columbia River in reach 3, which has a length of more than 200 river miles between Vernita Bridge and Warrendale. Most of these tributaries were monitored and, in aggregate, accounted for 91 percent of the gain in streamflow within the reach. The tributaries in reach 3 accounted for 74 percent of the TN discharged to the reach (excluding the John Day River), 92 percent of the TP, and 78 percent of the SS. These percentages can be viewed qualitatively by comparing the relative heights of the paired bar graphs for reach 3 in figures 12, 13, and 14. The major monitored tributaries discharging to this reach are the Yakima River, the Snake River, and the Deschutes River (table 8). In aggregate, the contributions of these three tributaries accounted for at least 94 percent of the TN, TP, and SS discharged to reach 3 from all of the monitored tributaries, with the Snake River by far the largest (table 9). As with reaches 1 and 2, point-source discharges of TN and TP to the main stem were small by comparison.
Reach 4 was different from the other reaches in that it had only minor tributary inputs, the most notable being the Sandy River (which was not included in the analysis). Reach 4 is affected primarily by point-source contributions (table 8). This reach spans about 41 river miles between Warrendale (immediately downstream of Bonneville Dam) and Marker 47 (located upstream of the confluence of the Willamette and Columbia Rivers). Within this reach, the point-source discharge of TN represented most of the nitrogen gain (figure 12), whereas the relative contribution for TP was notable, but smaller (figure 13).
Reach 5 began at Marker 47 and ended at Beaver Army Terminal (river mile 53.8). The gain within the reach for both TN and TP was less than the discharge of TN and TP from the Willamette River (figures 12 and 13). Without some in-stream loss or storage (for example, biological uptake or settling), the discharge of nutrients from the Willamette River should not have contributed more than the total of the gain in nutrients in this reach, especially since the tributary discharges from the Cowlitz River, Kalama River, and the Lewis River were not included in the analysis. Aside from the possibility of in-stream loss or storage, a reason for the discrepancy between TN and TP load and gain in this reach is not apparent. The discharge of SS from the Willamette represented slightly less than one third of the SS gain within the reach. This was to be expected considering the SS inputs from the other tributaries in this reach (particularly the Cowlitz River).
Four water-quality monitoring stations in Oregon were used to assess nutrient and sediment transport on the main stem of the Willamette River (fig. 15): Harrisburg, (RM 161, fig. 1, site 7), Albany (RM 119, fig. 1, site 11), Salem (RM 84, fig. 1, site 14), and Portland (RM 13, fig. 1, site 33). In addition, 6 water-quality stations on Willamette River tributaries and 20 point sources were used to assess nutrient and sediment load to the river.
The Willamette River discharged an annual mean load of 160,000 lb/d of TN (table 10), 16,300 lb/d of TP, and 2,780 ton/d of SS to the Columbia River in WY 2000 (on the basis of annual mean loads at Portland, Oreg.; fig. 1, site 33). These contributions represented discharges from all tributaries and point sources, ground-water systems, and the environmental processing of nutrients as Willamette waters flowed to the Columbia River. As was done with the main stem of the Columbia River, an accounting of streamflow gains was made for the reaches defined by the main-stem sites on the Willamette River (table 11). None of the reaches had tributaries that accounted for more than one-half the streamflow gain (table 11). Discharge of TN, TP, and SS increased steadily down the main stem, with the largest increase occurring in reach 3 between Salem and Portland (figs. 16, 17, and 18). Because they accounted for so little of the streamflow gain, the discharge of TN, TP, and SS from the six monitored tributaries to the Willamette River accounted for only a small part (within their respective reaches) of the gain in TN, TP, and SS in reaches 1 through 3 (figs. 16, 17, and 18).
Reach 1 spans a distance of 42 river miles and is bounded upstream by Harrisburg and downstream by Albany (fig. 17). About 10 river miles upstream from Harrisburg, the Willamette River receives point-source runoff from the Eugene/Springfield Water Pollution Control Facility and non-point source runoff from the McKenzie River. These two sources, with about equal annual TP loads, accounted for 59 percent of the annual mean TP load discharge at Harrisburg (table 10). Contributions of TN from the Eugene/Springfield WWTP represented 32 percent of the load at Harrisburg. In reach 1, the Willamette River receives urban runoff from Corvallis, as well as discharge from the Corvallis WWTP. It also receives nonpoint-source runoff from the Long Tom River, in addition to the Calapooya River and Mary’s River (the latter two of which were not included in the analysis). The annual load of TP discharged to the Willamette River from the City of Corvallis’ WWTP represented 27 percent of annual mean load in the Willamette River at Albany (about 5 times the annual mean load discharged from the Long Tom River), and 80 percent of the increase in TP annual mean load between Harrisburg and Albany. However, the annual TN load from the city’s WWTP was less than 2 percent of the annual mean load in this reach. Although tributary inflow increased streamflow by only 25 percent in the reach from Harrisburg to Albany, the tributaries, nonpoint-source runoff, and point source discharges were responsible for increasing the annual mean TN load by nearly 140 percent, increasing the annual mean TP load by about 11 percent, and increasing the annual mean SS load by 130 percent.
Reach 2 covered the 35 river miles between Albany and Salem, where annual mean streamflow increased by nearly 10,000 ft3/s owing to inputs from the Luckiamute River, the Santiam River, and several smaller streams (none were included in the analysis). Within reach 2, TN and TP from point sources discharging directly to the Willamette River were small and negligible in comparison to tributary discharges of TN and TP (table 10, fig. 16 and 17). Therefore, discharges from the Luckiamute River and the Santiam River, although not measured directly, were likely important sources of nutrients and SS to the Willamette.
In the more than 70 river miles from Salem to Portland (reach 3), the Willamette River receives discharges from point sources resulting from high-density urbanization in Portland and the surrounding metropolitan areas, as well as Salem. This reach also is affected by nonpoint sources, principally agricultural runoff. Annual mean streamflow in reach 3 increased by 11,300 ft3/s. About 45 percent of this increase was from monitored tributaries. This reach also received tributary inputs from the Yamhill and Molalla Rivers (not included in the analysis), Pudding, Tualatin, and Clackamas Rivers, in addition to several small creeks. In this reach, monitored tributaries and point sources accounted for nearly one-half the 85,600 lb/d gain of annual mean TN load and about 70 percent of the 7,000 pounds per day gain in annual mean TP load. The largest annual mean loads of TN were from the Tualatin River (16,600 lb/d) and the Pudding River (11,700 lb/d). Annual mean point-source loads within this reach (which did not include the City of Portland’s main WWTP) equaled almost 10,000 lb/d of TN. The largest point-source annual mean load of TN was 5,900 lb/d from the City of Salem’s WWTP. Of the tributaries with estimated SS loads, the Clackamas River was the largest monitored tributary in the entire Willamette basin. However, the estimated load from the Clackamas River may have been biased low because of interagency differences in field sampling protocols. Annual mean point-source loads within the reach equaled about 2,700 lb/d of TP.
The Snake River travels more than 1,000 river miles from its headwaters and drains parts of Oregon, Washington, Idaho, and Wyoming. Its drainage basin represents about 42 percent of the Columbia Basin’s drainage area. The uppermost site for load estimation was the Snake River at Flagg Ranch, WY. (RM 1,050, fig. 1, site 100); the lowermost site was the Snake River at Burbank, WA (RM 8.7, fig. 1, site 47). Two intermediate sites were at Snake River at Buhl, ID (RM 597, fig. 1, site 90) and Snake River at King Hill, ID (RM 546, fig. 1, site 91) [fig. 19].
The Snake River was a major source of nutrients and sediment to the Columbia River, discharging an annual mean load of 171,000 lb/d of TN, 13,300 lb/d of TP, and 1,220 ton/d of SS (table 12). Characterizing nutrient and sediment deliveries from tributaries to the Snake River was difficult because only 2 of the 32 major Snake River tributaries met the screening criteria for estimating loads. Although water-quality records were available for several tributaries, most water-quality data were only seasonal instead of the required coverage throughout the water year. The annual mean load of TN, TP, and SS increased 151 fold, 50 fold, and 9 fold, respectively, from the headwaters at Flagg Ranch to the mouth (fig. 20). In reach 2, between Buhl and King Hill, a distance of 51 mi, streamflow and TN discharge nearly doubled, TP increased by slightly more than one-third, and SS remained essentially unchanged. The Snake River between Buhl and King Hill received a large amount of nutrient-rich and sediment-free ground-water discharge, in addition to some phosphorus discharges from aquaculture activities. These sources could explain the patterns in streamflow, nutrients, and sediment observed in this reach.
Nutrient and sediment delivery to Puget Sound was assessed by using water-quality monitoring stations near the mouths of 8 major tributaries and estimated nutrient loads from 28 point sources. Figure 21 shows the location of water-quality stations on 8 major tributaries to the Sound and the 2 point sources that discharge directly to those tributaries (but not directly to the Sound). The water-quality stations were on the Duckabush River near Brinnon, Wash. (fig. 1, site 71), the Skokomish River near Potlatch, Wash. (fig. 1, site 61), the Deschutes River at East Street Bridge, Wash. (fig. 1, site 56), the Puyallup River at Meridan Street, Wash. (fig. 1, site 57), the Duwamish River at Golf course at Tukwilla, Wash. (fig. 1, site 67), the Snohomish River at Snohomish, Wash. (fig. 1, site 76), the Skagit River near Mount Vernon, Wash. (fig. 1, site 81), and the Nooksack River at Brennan, Wash. (fig. 1, site 84). The areas draining to these eight tributaries account for about 50 percent of the surface area of the Puget Sound basin. A study by Embrey and Inkpen (1998) determined that these tributaries delivered 65 percent of the surface-water inflow and accounted for about 75 percent of the inorganic nitrogen (nitrite plus nitrate nitrogen and ammonia nitrogen) and TP loads to the Puget Sound. In WY 2000, the total discharge of these tributaries to Puget Sound equaled 61,000 lb/d of TN, 13,000 lb/d of TP, and 11,500 ton/d of SS (fig. 22).
Figure 22 shows the WY 2000 monitored tributary and estimated point-source loads of TN, TP, and SS discharged to the Puget Sound. The contribution of nutrients to the eight monitored tributaries from point sources located within their catchments was very small (six tributaries had no direct point-source contributions and the estimated point-source contribution to the other two tributaries was never more than 6 percent of the predicted in-stream load for WY 2000). However, the sum of the nutrient loads from point sources discharging either directly to the Puget Sound or very close to the mouths of tributaries was comparable to the sum of the monitored tributary loads. Point sources contributed almost 50 percent of the annual TN and TP load discharged to the Sound in WY 2000, with WWTPs accounting for more than 90 percent of the point-source load. The largest tributary loads of TN in WY 2000 were from the Snohomish River and the Skagit River. Together, they accounted for about 57 percent of the total monitored tributary TN load to the Sound. With the addition of the TN load from the Nooksack River, the contribution increased to 77 percent of the total TN load. The Skagit River was the single largest tributary contributor to the Sound for TP, accounting for more than 31 percent of the monitored tributary load. These tributaries are on the east side of the Sound, in areas where the intensity of urbanization and agricultural activity is greatest.
Monitored WY 2000 tributary nutrient loads to the Puget Sound were only a fraction of those discharged from the Columbia River. The TN and TP loads from all of the monitored Puget Sound tributaries were equal to about 11 percent and 24 percent of the TN and TP load, respectively, at the Columbia River at Beaver Army Terminal, Oreg. (fig. 1, site 44). The smaller loads are because Puget Sound’s drainage area is only one-seventh of the Columbia River Basin and the sum of the mean streamflow for the eight monitored tributaries in WY 2000 was about 17 percent of the value in the Columbia River at Beaver Army Terminal. However, the total annual mean SS load discharged from all monitored tributaries to the sound (fig. 22) represented about three-quarters of the annual mean SS load discharged from the Columbia River.
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