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Water quality benefits of wetlands under historic and potential future climate in the Sprague River Watershed, Oregon




Records, Rosemary M., author
Fassnacht, Steven, advisor
Arabi, Mazdak, advisor
Duffy, Walter, committee member
Butters, Greg, committee member

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An understanding of potential climate-induced changes in stream sediment and nutrient fluxes is important for the long-term success of regulatory programs such as the Total Maximum Daily Load and sustainability of aquatic ecosystems. Such changes are still not well characterized, particularly in the Pacific Northwest, although shifts in stream flow associated with warming temperatures have already been observed in the region. Conservation practices such as wetland restoration are often regarded as important in watershed-scale management of water quality. However, the potential of wetland gains or losses to alter future stream water quality conditions has received relatively little study. The primary goal of this research is to assess the basin-scale regulation of sediment, nitrogen and phosphorus provided by variable wetland extent under current climate and potential mid-21st century climate. Specific objectives of the study are (1) to evaluate the effects of present-day wetlands on stream water quality under current climate; (2) to identify direction and magnitude of potential changes in stream flow, sediment, and nutrient loads under present-day wetlands and potential future climate; and (3) to determine how wetland gain or loss might exacerbate or ameliorate climate-induced changes in future water quality. These objectives are investigated with the Soil and Water Assessment Tool (SWAT) hydrologic model in the Sprague River watershed in southern Oregon, United States, which has been historically snowmelt dominated and where elevated nutrient loads in the 20th century have contributed to decline of fish species downstream. Results suggest that present-day wetlands under current climate may result in substantially lower nitrogen and phosphorus loads at the Sprague River watershed outlet. SWAT simulations forced with precipitation and temperature from six General Circulation Model (GCM) derived climate projections for 2030-2059 suggest uncertainty in magnitude and direction of both precipitation and stream flow changes on an average annual and monthly basis. Under present-day wetland extent, long-term average annual runoff for 2030-2059 decreased by 4% under one projection relative to a baseline period of 1954-2005, but increased by 6-31% under other projections. However, change in future annual runoff was statistically different from baseline for only two of six climate projections. Late spring and summer stream flow was lower in all simulations but significantly different from baseline in only some cases; for simulations driven with wetter future climate projections average monthly flow increased significantly from approximately October through March, and peak average monthly flow increased from 3-36% but timing did not alter. A simulation driven with a drier future climate projection showed decreases in average flow for most months, but was not significantly different from baseline. Simulated average annual sediment and nutrient loads generally tracked flow seasonality and decreased by 6% (sediment), 8% (TN) and 11% (TP) under one projection, but increased from 7-52% (sediment), 4-37% (TN) and 1-38% (TP) under other projections. Findings suggest that nutrient loads at the Sprague River outlet under future climate and scenarios of wetland change could vary significantly from baseline, or could be similar to the historic period. However, a threshold of wetland loss may exist beyond which large increases in nutrient loads could occur, and wetland gain might do little to ameliorate climate impacts to stream water quality in the Sprague River watershed.


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