Olson, Patricia Lee. 1996. Shallow subsurface flow systems in a montane terrace-floodplain landscape: Sauk River, north Cascades, Washington. Ph.D.
Constant Channel, a salmonid spawning and rearing channel in the Sauk River floodplain, provides an opportunity to study subsurface flow in a montane floodplain and terrace landscape. The conventional viewpoint on subsurface flow in riverine floodplains assumes an allvial aquifer underlies the surface and aquifer water levels are controlled by river stage. This study emphasizes the groundwater system viewpoint. Three distinct sources of subsurface flow to Constant Channel are proposed: hillslope-upper terrace, lower terrace, and Sauk River. The hypotheses are tested using time-series and discriminant analysis statistical techniques on: (1) hydrologic data, including surface and subsurface water levels, seepage quantities, and precipitation; (2) subsurface and surface temperatures of water, air, and soil; and (3) dissolved chemical constituent data including cations, anions, and nutrients. The three data sets are treated as separate case studies to independently test the hypotheses.
A conceptual framework describing predicted flow routing and regimes to Constant Channel is developed. The framework is based on generally accepted experimental studies, field observation, or numerical models. Although floodplain and terrace morphology are complex, the study's underlying premise is subsurface flow will behave in a predictable manner. Key factors of hydrologic routing and regimes are expected to vary predictably with source areas. Factors are: (1) location of recharge areas to discharge areas; (2) potential storage volume; (3) baseflow recession and duration; (4) specific discharge of subsurface flow to Constant Channel; (5) direction of flow; and (6) storm response. Factors 1-3 define water availability and drainage capacity. Factors 4-6 describe the source area transport 'efficiency'.
Three source groupings and three seasons are significant discriminators for explaining variance in hydrologic regimes of Constant Channel catchment. Primary and secondary topography, soil drainage characteristics, remnant fluvial features, and spatial continuity adequately delineate distinct sources of subsurface flow that supports the hypotheses. Valley side and floor relief were dominant controls on the intermediate flow systems' direction. Terrace and floodplain secondary topography influence the direction of shallow local flow. Seasonal climatic patterns determine the quantity and timing of flow to the channel.