Effects of agricultural drainage ditch maintenance on water temperature in the Snoqualmie River valley, Washington State

Abstract

Water temperature profoundly affects the health of freshwater ecosystems and is an increasing water quality concern in the Pacific Northwest, where warm stream temperatures can threaten the survival and growth of already endangered salmonid species. Agricultural drainage ditches exist to improve drainage off fields but are also used by endangered salmon. Maintenance of these waterways is regulated by the Washington Department of Fish and Wildlife because it may affect fish survival and habitat, but the impacts are not well studied. This study examines the effect of maintenance (dredging sediment and reed canary grass then planting a buffer of native riparian plants) on water temperature in 9 drainage ditches between July and early October 2011. Weekly and continuous water temperature measurements were collected in the Snoqualmie Agricultural Production Districts in Washington and were analyzed for the 7-day average of the daily maximum temperature, water temperature sensitivity to air temperature, and heating from upstream to downstream. Vegetation above and within the waterway was measured to examine canopy effects on water temperature and reed canary grass presence. Maintenance appears to have the greatest influence on water temperature immediately after it is performed, likely because maintenance removes all vegetative cover and exposes waterways. Canopy coverage over previously maintained ditches did not show a relationship with maximum water temperatures, but it did appear to reduce reed canary grass presence. The primary source of the water (valley wall-based seep, shallow groundwater interception, oxbow lake, or surface tributary) appears to have a significant influence on water temperatures for these agricultural drainage ditches, perhaps more so than maintenance activities over time.