A LARGE LATE PLEISTOCENE OUTBURST FLOOD FROM PLUVIAL LAKES ALVORD AND COYOTE INTO THE OWYHEE RIVER, OREGON

A large late Pleistocene flood traveled into the Owyhee River as a result of a rise and overflow of pluvial Lakes Alvord and Coyote in southeastern Oregon. Lake Alvord spilled over its eastern rim through Big Sand Gap and into the adjacent Coyote Lake basin after reaching an altitude of at least 1300 meters. Lake Coyote then rose rapidly to 1295 meters, overtopped its northern and eastern rims, and released a flood volume of ~29 km³ of water down the Crooked Creek drainage and into the Owyhee River. Geomorphic and sedimentological evidence of the flood abounds in the Crooked Creek drainage and includes deeply incised canyons, stripped bedrock surfaces, and many boulder bars containing imbricated clasts up to 4 meters in diameter and up to 40 meters above the present channel. Strongly developed shorelines in the Alvord and Coyote Basins at 1280 meters and 1277 meters, respectively, suggest that the lakes were separated by a sill at 1280 meters following the flood. In the Coyote Basin, this lower shoreline grades to the incised outlets, indicating that postflood overflow may have connected the Owyhee River (of the Columbia River drainage) to Lake Coyote for an extended period of time, possibly opening a migration corridor for aquatic species.

At both of the Lake Coyote outlets, critical depth calculations show a maximum combined outflow of 70,000 m³s^-1. The value accords well with step-backwater calculations indicating a minimum flow of 60,000 m³s^-1 20 kilometers downstream where the overflow was confined to a single channel. Further downstream Crooked Creek drainage flows through a deep gorge that could not have contained all of the flow at 60,000 m³s^-1, resulting in the flood overtopping divides and traveling down numerous tributaries and into the Owyhee River.

Directly dating the flood is in progress using ³He cosmogenic dating on basaltic flood boulders. These dates will be supported by radiocarbon dates of shoreline features in both the Coyote and Alvord lake basins.