Hydrologic and ecologic impacts of climate change in the Grande Ronde River Basin

David E. Rheinheimer

A key component of the River Continuum Concept, which conceptualizes a riverine ecosystem as a continuous gradient from headwaters to mouth, is the physical template, including climate and hydrologic regime. The Grande Ronde River basin in northeastern Oregon is characterized by snowfed streams from the Wallowa Mountains in the southern part of the basin and from the Blue mountains in the north and west. Precipitation as snowfall in the colder months (January through May) melts during the springtime, resulting in peak flows in the spring and low flows around September. There is little precipitation during the hotter summer months. The climate is expected to change, however, resulting in an altered hydrologic regime. Predictions indicate a general increase in temperature in the Pacific Northwest over the next few decades, consistent with past observations. While precipitation is generally predicted to rise in the Pacific Northwest, it has in fact decreased in the Grande Ronde region over the past few decades. Climate change is predicted to result in reduced snow water equivalent, earlier peak flows, higher colder month flows, and lower warm month flows. Both changes in climate and the hydrologic regime will change vegetation patterns, which further affects the local water cycle. These changes will alter the physical template upon which the biological community depends, but will not alter the underlying assumptions of the River Continuum Concept.