A physical and chemical characterization of stream water draining three Oregon Coast Range catchments

Graduation date: 2008Few studies have examined both long-term and fine-scale spatial variations in\ud water quality of small streams in the Pacific Northwest. As such, a case study was\ud conducted to determine if current physical and chemical properties of water in three\ud streams located in the Oregon Coast Range differed from historically measured\ud conditions, taking differences in past management regimes into account. In addition,\ud this research provides an assessment of spatial and temporal variability in nitratenitrogen\ud (N) concentrations and summer stream temperatures within each catchment.\ud The three research catchments were part of the Alsea Watershed Study (1959-\ud 1973), where effects of forest management practices were examined using a pairedwatershed\ud study design. One catchment, Needle Branch, was clear-cut with no\ud protection provided to the stream. Harvesting in Needle Branch was followed by an\ud intense broadcast burn to remove logging slash. Another catchment, Deer Creek, was\ud patch-cut in three small units resulting in a 25% harvest of the total catchment area,\ud but buffers were retained along fish-bearing streams. The third catchment, Flynn\ud Creek, was used as a control.\ud In this revisit to the Alsea Watersheds, measurements were conducted\ud continuously (discharge, turbidity), intermittently (suspended sediments), and at\ud regular intervals (nitrate-N) for one year between October 2005 and September 2006.\ud Summertime stream temperature was also measured every half-hour from mid-June to\ud mid-September.\ud Comparisons of recent data with historic data show no detectable changes over\ud time for streamflow characteristics (annual runoff volume, peak flow discharges, and\ud number of low-flow days), annual sediment yield, or summer maximum stream\ud temperatures. Current nitrate-N export was similar to historically measured values for\ud Flynn Creek and Deer Creek; however, export at Needle Branch was increased over\ud past levels. This observation may be caused by dense colonization of the riparian area\ud with red alder (Alnus rubra), a N-fixing species, following the 1966 harvest. Patterns\ud of nitrate-N concentration varied throughout each catchment and are likely influenced\ud by the current distribution of red alder stands. Synoptically measured stream\ud temperatures were variable along each stream’s longitudinal profile. The ability to\ud meet Oregon’s water quality standard for temperature was dependent on measurement\ud location and method of analysis. Evaluating individual sampling points as discrete\ud records resulted in each stream exceeding the standard for at least one measurement\ud location, whereas evaluating the criteria based on the mean of all data collected within\ud the mainstem stream excluded Flynn Creek and Needle Branch from violation. These\ud results highlight the physical and chemical variability of stream water draining Oregon\ud Coast Range headwater catchments and provide insight as to where future work\ud should be focused to gain a more thorough understanding of these dynamic systems