IMPACTS OF BASALTIC VOLCANISM ON INCISED FLUVIAL SYSTEMS: DOES THE RIVER GIVE A DAM?

The incursion of large volumes of basaltic lava into a fluvial landscape results in a fascinating array of intertwined volcanic and fluvial features that can chronicle and constrain landscape evolution for millions of years. A characteristic suite of morphologic and stratigraphic features document the nature and geologic impact of the focused interaction between lava and water. The ingress of a stream of molten rock into an active, incised fluvial system can result in diversion; temporary blockage and catastrophic flooding; or long-term damming and sediment trapping. The latter two, in particular, can facilitate the delivery of large volumes of exogenous material into the channel and valley downstream from the point of blockage. This ultimate act of valley bottom re-paving and filling obliterates pre-lava topography and can elevate long reaches of the valley floor by up to 100s of meters. Repeated lava flows can create and maintain a detailed chronicle of river incision and landscape evolution. The long history of lava flows in the Grand Canyon is a prime example that has received much scrutiny. A comparably long record along the Owyhee River of southeast Oregon is also a prime example that has only recently been explored in detail. A >60 km reach of the Owyhee has been dammed and partially filled by at least 6 basaltic lava flows over the last 2 million years. Lacustrine strata linked to Owyhee lava-dams suggest that some dams may have persisted for 10,000s of years. This stands in stark contrast to some lava-dams in the Grand Canyon that appear to have undergone catastrophic failure soon after or even during emplacement. The protracted process of lava dam removal along the Owyhee involved direct incision through intracanyon lava in narrow reaches confined by steep, resistant canyon walls and by channel relocation and incision along contacts between intracanyon lavas and weaker rocks in wider parts of the valley. The latter mode has an additional, major geomorphic impact by controlling the occurrence, style, and scale of landsliding on both sides of the valley. Many landslides have blocked the river, failed catastrophically, and generated large floods.