THE SOURCE OF THE RIVER: GEOLOGIC CONTROL OF THE HYDROLOGIC REGIME, WILLAMETTE RIVER, OREGON

At the scale of large river basins, such as the Willamette River in western Oregon, we have an imperfect picture of streamflow generation mechanisms, their seasonality, and spatial distribution, and underlying geological controls. We used long-term USGS streamflow data to characterize natural streamflow regimes within the Willamette River basin, at spatial scales ranging from individual mountain streams to larger rivers, such as the McKenzie and mainstem Willamette. Using a new hydrograph separation technique, we compared peak and low streamflows from streams draining both the older, dissected terrain of the Western Cascades, and younger, undissected volcanic terrains of the High Cascades.

Flow regimes clearly demonstrate underlying geologic control, with streams from the younger volcanic region exhibiting less pronounced peak streamflows and more sustained base flow, than streams draining the older platform. Analysis of streamflow regimes for the mainstem Willamette similarly revealed strong seasonal variations in the source of water, in relation to the underlying geology and topography of contributing tributaries. Contrary to popular belief, most summer flow out of the High Cascades is not due to snowmelt. Instead, the High Cascades are a vast hydrologic sponge that stores many decades worth of water as deep groundwater. This water emerges from springs along both the east and west flanks of the Cascade crest, fed by very large regional aquifers flowing through the young volcanic rock. Even during drought years, creeks and rivers fed by groundwater flow at virtually constant discharges. Operation of large flood control dams modify the natural flow regime by shifting it towards the High Cascade end-member.

Differences in streamflow are also reflected in distinctive sediment transport regimes, stream temperatures, and channel morphology. High Cascade streams have very low bedload and suspended sediment transport rates, lack well-defined bedforms and floodplains, and, in forested reaches, are typically choked with stable wood accumulations and jams. Western Cascade streams have much higher wood and sediment transport rates, well-defined bedforms and floodplains, and less stable wood accumulations suggesting more frequent transport.