Book Description

Two small adjacent watersheds on the west-central edge of the Willamette Valley of western Oregon were studied. Information was gathered on existing conditions to determine the suitability of the two watersheds for a paired-watershed study design. A soil survey was conducted using slightly modified soil series previously established for the area. Distributions of the major soil series in the two watersheds were compared. Four transects were sampled using extracted soil cores and hand augering. Results of the transects closely paralleled pre-existing geomorphic schemes. Land-use patterns of the two watersheds were identified and compared. Streamflow and suspended sediment levels of the two watersheds were monitored and the effects of eight storms were compared. The soil survey revealed more diversity in the number of major soil series and greater percentages of heavier textured soils in watershed D3. Most of its forestland occurred on heavy textured soils, whereas most of the forestland in watershed 04 occurred on moderate textured soils. Watershed D3 had almost twice the percentage of cultivated wheatland as watershed D4, whereas D4 had greater percentages of area in grassland. These differences suggest that the soils of D3 might have lower infiltration and permeability than the soils of D4, conditions which are more conducive to lower baseflows and greater runoff and erosion. D3 has a fairly extensive tile drain network and D4 has three settling basins. The impact of these management structures is not apparent. Streamflow, suspended sediment levels, and precipitation for eight storms were compared. Comparisons of hydrographs indicated different response times in storm hydrograph rise for D3 and D4. Determination of these delay factors and adjusting streamflow values accordingly revealed a highly significant relationship between the streamflow levels, indicating a consistent hydrologic relationship between D3 and D4. D3 had consistently lower baseflow levels than D4. Comparisons of suspended sediment levels revealed consistently higher sediment levels in D3 which resulted in greater total yields in all eight storms. These results indicate an unequal but consistent hydrologic relationship between the two watersheds and that they are suitable for a paired-watershed study design.