Geologic Controls on Physical Habitat Distribution, Grande Ronde River, Oregon and Washington

Dylan J. Caldwell

The Grande Ronde River, and its associated watershed, flows through a region in northeastern Oregon, which has experienced a dynamic geologic history. During Mesozoic times, the granitic Wallowa Pluton intruded into the area. Approximately 17.5 Ma (million years ago), during Miocene times, the eruption of the Columbia River Basalt Group commenced and proceeded for another 11.5 million years. These flood basalts and associated tectonic events drastically changed the surrounding terrain. Significant amounts of uplift continued after the basaltic eruptions and caused local river systems to develop antecedent features that flow through distinctly different rock types. The River Continuum Concept (Vannote et al. 1980) may provide a basis for understanding the gradational changes of ecological habitats seen in the Grande Ronde River system. However, due to the geologic complexity of the area, the Process Domain Concept (Montgomery 1999) may prove more applicable for understanding the distribution of physical habitat and ecosystem function in this locality.