Relations Among Floodplain Water Levels, Instream Dissolved-Oxygen Conditions, and Streamflow in the Lower Roanoke River, North Carolina, 1997 – 2001
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The lower Roanoke River corridor in North Carolina contains a floodplain of national significance. Data from a network of 1 streamflow-measurement site, 13 river-stage sites, 13 floodplain water-level sites located along 4 transects, and 5 in situ water-quality monitoring sites were used to characterize temporal and spatial variations of floodplain and river water levels during 1997 – 2000 and to describe dissolved-oxygen conditions in the lower Roanoke River for the period 1998 – 2001.
Major differences in the relation of floodplain inundation to flow occurred both among sites at a given transect and among transects. Several floodplain sites were inundated for the full range of flow conditions measured during the study. These included one site on the Big Swash transect (at about river kilometer 119); one site on the Broadneck Swamp transect (river kilometer 97), which was inundated 91 percent of the time during the study; one site on the Devils Gut transect (river kilometer 44), which was inundated throughout the study; and three sites on the Cow Swamp transect (near river kilometer 10).
The relation of floodplain inundation depth to Roanoke River flow was highly variable among sites. There was no relation between flow and inundation depth at one of the Big Swash sites or at any of the four Cow Swamp sites. At two of the Big Swash transect sites, there was some relation between inundation depth and 10-day mean flow for flows greater than 700 cubic meters per second. A relatively strong relation between inundation depth and 10-day mean flow occurred at two of the Broadneck Swamp sites and, to a lesser degree, at two of the Devils Gut transect sites.
There was much greater interannual variability in floodplain water levels, as represented by the difference between the maximum and minimum daily water level for a given calendar date during January – May and September – October than during the summer and late fall months. If data from this study are representative of long-term conditions, then this means that there is less uncertainty about what future floodplain water levels will be during June – August and November – December than during other months.
Rates of ground-water decline, primarily due to evapotranspiration, were fairly similar at all sites, ranging from about 3 to 4 centimeters per day. For a 10-day mean flow of 300 cubic meters per second, an evaporative loss of 2 centimeters per day is equal to about 56 cubic meters per second. Evapotranspiration rates are much lower during the fall and winter months, so losses of river flow to floodplain processes likely are much lower during those months.
The ground-water gradient at most sites was from the floodplain to the river, indicating a potential for ground-water movement into the river from the floodplain. At two of the Devils Gut sites, however, the water level often was higher in the river than in the floodplain when floodplain sites were not inundated. This indicates that there is a potential for river water to move as ground water from the river into the floodplain. It seems likely that this feature observed at the Devils Gut transect occurs elsewhere in the lower Roanoke River corridor.
Dissolved-oxygen concentrations typically decrease with increasing distance from Roanoke Rapids Dam. During the 1998 – 2001 study period, the median dissolved-oxygen concentration at Halifax (river kilometer 187), the upstream-most station, was 8.4 milligrams per liter, and the median concentration at the downstream-most station (NC-45, bottom sensor; river kilometer 2.6) was 6.6 milligrams per liter. Several synoptic measurements of dissolved-oxygen concentration down the river identified the presence of a dissolved-oxygen sag in the vicinity of Halifax, with some recovery of concentrations between Halifax and about Scotland Neck at river kilometer 156. Data from the synoptic measurements also indicated that the greatest rate of dissolved-oxygen change with distance along the river was downstream from Hamilton (river kilometer 97).
The frequency with which the North Carolina water-quality standards for dissolved oxygen were exceeded also increased with distance from Roanoke Rapids Dam, and many of the low dissolved-oxygen events were concurrent with backswamp drainage. The number of days during the study period for which the daily mean dissolved-oxygen concentration was less than 5 milligrams per liter was: Halifax — 2 days; Oak City — 18 days; Grabtown — 45 days; Jamesville — 136 days; and NC-45 bottom — 235 days. Most of these occurrences were during the months of May – October, with the most in September. If the low dissolved-oxygen concentrations associated with Hurricane Floyd flooding were not considered, the month during which daily mean dissolved oxygen was most likely to be less than 5 milligrams per liter was June — typically the month during which the higher spawning-enhancement flows are stepped down to the lower, summer load-following flows. It is likely that this change in flow regime and the associated draining of the backswamps is at least partially responsible for the relatively large number of occurrences of low dissolved oxygen in June. It also is worth noting that during the study period, monthly point-source biochemical oxygen demand loads in the summer were typically one-third of the loads during the winter.
Dissolved-oxygen concentrations are qualitatively related to flow conditions. Daily mean dissolved-oxygen concentrations less than 5 milligrams per liter occurred only at 10-day mean flows (measured at Roanoke River at Roanoke Rapids) of 180 – 280 cubic meters per second at Oak City, only at 10-day mean flows less than 240 cubic meters per second at Grabtown, and primarily at 10-day mean flows less than 300 cubic meters per second at Jamesville. At the NC-45 site, under current loading conditions, it is unlikely that the bottom daily mean dissolved-oxygen concentration would be less than 5 milligrams per liter for 10-day mean flows greater than about 290 cubic meters per second. Likewise, at a 10-day mean flow of 200 cubic meters per second, bottom daily mean dissolved-oxygen concentrations could be as low as, but probably no lower than, 3 milligrams per liter. The difference between mid-depth and bottom dissolved-oxygen concentrations at the NC-45 site also are related to flow in somewhat the same manner as daily mean dissolved-oxygen concentrations. At 10-day mean flows less than 200 cubic meters per second, the difference between mid-depth and bottom dissolved-oxygen concentrations is likely to be greater than 1 milligram per liter; at 10-day mean flows less than 100 cubic meters per second, the difference is likely to be 2 milligrams per liter. For both flows, the difference is as likely to be positive (mid-depth greater than bottom) as negative (mid-depth less than bottom).
During May 11, 2000 – December 31, 2001, when data from both the mid-depth and bottom sensors were available at the NC-45 site, the median bottom dissolved-oxygen concentration was 6.1 milligrams per liter, and the median mid-depth concentration was 6.1 milligrams per liter. Hourly dissolved-oxygen concentrations less than 4 milligrams per liter occurred about four times more often at the bottom sensor than at the top, but hourly dissolved-oxygen concentrations greater than 8 milligrams per liter occurred about 50 percent more frequently at the bottom than at mid-depth. Dissolved-oxygen concentrations were about as likely to be higher at the bottom sensor as at the mid-depth sensor.
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