HYDROGEOCHEMICAL CONTROLS ON FELSIC VOLCANIC ROCK WEATHERING ACROSS A CLIMATIC GRADIENT: CENTRAL CASCADES AND HIGH DESERT, OREGON

Investigations into watershed scale hydrogeochemistry are an important component to the study of mineral weathering rates and fluxes from rivers to oceans. The Upper Deschutes watershed was chosen to study because of the variable volcanic lithology and changing climate to the east. The watershed is comprised of two distinct regions, the Cascades and the High Desert. The Cascade region drains the crest of the Cascades to the east and is comprised of Miocene to Quaternary age volcanic rocks that range in composition from basalt to rhyolite. The High desert region drains the Ochoco Mountains via the Crooked River and surface geology in the study area is primarily composed of Eocene and younger volcanic and volcanic derived sedimentary rocks. Soils formed within the Deschutes watershed tend to be weakly developed with minimal B-horizon formation.

Stream and springs within the watershed were sampled for elemental analysis (majors and trace), discharge measurements were taken when appropriate, and springs were sampled for age dating by chlorofluorocarbon analysis. Lithologic distinctions within the drainage areas are difficult to detect with major elements for the majority of samples due to difficulty isolating discrete lithologic regions for sampling. Preliminary data show that smaller High Desert streams have similar elemental ratios as the central Cascade streams, however the total concentration for the majority of elements is higher. The higher solute concentrations are primarily due to evapoconcentration within the streams and lithologic differences between the Cascades and Ochocos. The larger streams show evidence of anthropogenic pollution such as high Na, Cl and SO₄ concentrations.

The data suggest that the Deschutes River watershed is a weathering limited system. Age constraint data from this study as well as others shows spring waters within the western Upper Deschutes basin to have relativity young recharge ages. The waters are also undersaturated for minerals present within the local bedrock and long-term data gathered from USGS gauging stations show an inverse relationship between element concentrations and discharge. Taken together, the young ages and geochemical data on the springs as well as discharge relationships within the streams indicate a quick-moving, weathering-limited system.