GIS-BASED ANALYSIS OF DRAINAGE REVERSAL IN THE PEND OREILLE RIVER, NORTHEASTERN WASHINGTON

Stradling and Kiver (1989) noted that tributaries to the Pend Oreille River in northeastern Washington are barbed (that is, they form acute downstream angles with the trunk stream), indicating flow reversal in the recent past (latest Pleistocene or Holocene time). Modern Pend Oreille River flows west out of Lake Pend Oreille, Idaho, then north through Washington and southern Canada before spilling into the Columbia River, which flows southeast to the Pacific Ocean. In Washington, however, most tributaries flow south out of the surrounding highlands then make abrupt U-turns northward as they join the Pend Oreille River. Another supporting indicator of flow reversal is a downstream narrowing of the river channel, which is opposite the normal pattern of downstream widening. These characteristics likely developed during some past episode of southerly flow in the main channel. We present results from a GIS-based analysis of the river system and surrounding landforms and discuss flow reversal relative to the glacial, lacustrine, and Ice-Age flood history of this region.