- The purpose of this study was to develop a mass-balance model for total suspended solids in the Tualatin River in order to better understand the clarity-turbidity problem in the river. Major sources and sinks of suspended solids in the river were identified, and seasonal effects were explored. The study also examined relationships between suspended solids and transparency, turbidity, chlorophyll i! in an attempt to better understand processes occurring in the river and its watershed.
To perform the mass balance, the river was divided into twelve sections, based on the monitoring stations of the Unified Sewerage Agency (USA) of Washington County, Oregon. Tributaries were treated as point sources flowing into one of these sections. The water quality and flow data of USA formed the basis of the mass balance, with additional flow data provided by the Oregon Water Resources Department, Tualatin Valley Irrigation District, and U. S. Geological Survey, and additional water quality data from Oregon Department of Environmental Quality.
Tributaries were found to be the major contributors of suspended solids loading in the Tualatin River. The major tributaries in this regard were Dairy Creek, Fanno Creek, Gales Creek, Rock Creek and Scoggins Creek. For the year 1992, the above five tributaries contributed 90% of the average suspended solids mass loading during the non-summer period and 79% during the summer season. Gales Creek is the major contributor to suspended solids mass loading during the non-summer season. Scoggins Creek, which receives the discharge of Hagg Lake is the major contributor of suspended solids to the river in the summer period (more than 50% of the combined loading of the five major tributary creeks, summer 1992). The tributaries also accounted for 63 % of thy flow (including withdrawals) in the river during the summer of 1992 and for 84% during the 1992 non-summer season.
Changes in suspended solids loading in the Tualatin River were computed at stations above and below the entries of Scoggins, Gales, Rock and Dairy Creeks. Major increases were observed for these tributaries, emphasizing the finding that tributaries contribute suspended sediment to the river during the entire year and are major contributors during the non-summer season (except Scoggins, higher contributor during summer).
The seasonal variation of the suspended solids loading in the river differed by as much as a factor of ten, the loading being lower in summer when suspended solids concentrations averaged about 50% of non-summer values. Water clarity was found to be higher in summer, during which time chlorophyll g concentrations were also higher. Suspended solids concentration was inversely correlated with transparency (water clarity) and directly correlated with turbidity but found to be unrelated to chlorophyll g concentration, indicating the algae were not a primary constituent of the total solids.
Increased chlorophyll i! concentrations were not found to relate to any one particular factor but were found to be related as a combination of air temperature, and total phosphorus concentrations. This indicates that the cause of algal blooms are due to a combination of factors especially nutrient levels, water temperature and the residence time of water in the quiescent pool area.