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Scientific Investigations Report 2011–5040

Relation between Flows and Dissolved Oxygen in the Roanoke River between Roanoke Rapids Dam and Jamesville, North Carolina, 2005–2009

By Loren L. Wehmeyer and Chad R. Wagner

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ABSTRACT

The relation between dam releases and dissolved-oxygen concentration, saturation and deficit, downstream from Roanoke Rapids Dam in North Carolina was evaluated from 2005 to 2009. Dissolved-oxygen data collected at four water-quality monitoring stations downstream from Roanoke Rapids Dam were used to determine if any statistical relations or discernible quantitative or qualitative patterns linked Roanoke River in-stream dissolved-oxygen levels to hydropower peaking at Roanoke Rapids Dam.

Unregulated tributaries that inundate and drain portions of the Roanoke River flood plain are crucial in relation to in-stream dissolved oxygen. Hydropower peaking from 2005 to 2009 both inundated and drained portions of the flood plain independently of large storms. The effects of these changes in flow on dissolved-oxygen dynamics are difficult to isolate, however, because of (1) the variable travel time for water to move down the 112-mile reach of the Roanoke River from Roanoke Rapids Dam to Jamesville, North Carolina, and (2) the range of in-situ conditions, particularly inundation history and water temperature, in the flood plain.

Statistical testing was conducted on the travel-time-adjusted hourly data measured at each of the four water-quality stations between May and November 2005–2009 when the weekly mean flow was 5,000–12,000 cubic feet per second (a range when Roanoke Rapids Dam operations likely affect tributary and flood-plain water levels). Results of this statistical testing indicate that at the 99-percent confidence interval dissolved-oxygen levels downstream from Roanoke Rapids Dam were lower during peaking weeks than during non-peaking weeks in three of the five years and higher in one of the five years; no data were available for weeks with peaking in 2007. For the four years of statistically significant differences in dissolved oxygen between peaking and non-peaking weeks, three of the years had statistically signficant differences in water temperature. Years with higher water temperature during peaking had lower dissolved oxygen during peaking. Only 2009 had no constistent statistically significant water-temperature difference at all sites, and dissolved-oxygen levels downstream from Roanoke Rapids Dam during peaking weeks that year were lower than during non-peaking weeks.

Between 2005 and 2009, daily mean dissolved-oxygen concentrations below the State standard occurred during only 1 of the 17 (6 percent) peaking weeks, with no occurrence of instantaneous dissolved-oxygen concentrations below the State standard. This occurrence was during a 9-day period in July 2005 when the daily maximum air temperatures approached or exceeded 100 degrees Fahrenheit, and the draining of the flood plains from peaking operations was followed by consecutive days of low flows.

First posted March 15, 2011

For additional information contact:
Director
USGS North Carolina Water Science Center
3916 Sunset Ridge Road
Raleigh, NC 27607
http://nc.water.usgs.gov/

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Suggested citation:

Wehmeyer, L.L., and Wagner, C.R., 2011, Relation between flows and dissolved oxygen in the Roanoke River between Roanoke Rapids Dam and Jamesville, North Carolina, 2005–2009: U.S. Geological Survey Scientific Investigations Report 2011–5040, 29 p.



Contents

Abstract

Introduction

Purpose and Scope

Description of the Study Area

Methods

Relation between Flows and Dissolved Oxygen in the Roanoke River

Effects of Tributary Inflows on In-stream Dissolved Oxygen

Characterization of Streamflow

In-Stream Dissolved-Oxygen Conditions

Summary and Conclusions

References Cited


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