FLUVIAL RESPONSES TO BASE-LEVEL LOWERING: EXAMPLES FROM THE TRUCKEE AND WALKER RIVERS, WESTERN NEVADA, USA

Rectified aerial photographs are used to document the historic evolution of the Truckee and Walker rivers that terminate in Pyramid and Walker lakes, respectively. These endorheic lakes represent base level for the rivers and both have undergone large declines in their lake-surface elevations because of upstream water consumption, which have caused lengthening, incision, and widening of the river trenches. In the case of the Truckee River, a net drop of 22 m in the elevation of Pyramid Lake since the late 19^th century has resulted in the lengthening of the river channel by ~ 4-5 km and a head cut that has migrated ~ 20 km upstream where it has been arrested by a small dam and bedrock in the channel. Marble Bluff Dam (MBD) was constructed in 1975 about 1.5 km upstream from the historic highstand shoreline (HHS; 1183 m) to arrest further incision. Below MBD and in the vicinity of the HHS incision has increased to ~ 20 m in the last 30 years, resulting in a series of terraces and a relatively flat gradient for the lowermost 5 km of the Truckee River channel. In contrast, the elevation of Walker Lake has fallen by ~ 50 m since the late 19^th century, which has resulted in the lower Walker River (LWR) channel extending ~ 25 km downstream from the HHS (1252 m) and a head cut migrating ~ 3 km upstream. In this case, the head cut has been arrested by a 1 m diameter steel pipe lying across the bed of the channel. Removal of this pipe by natural (e.g., floods) or human causes will undoubtedly lead to continued head cut migration. The long profile of the LWR prescribes a pronounced convex-up shape with maximum incision of about 10-12 m adjacent to and below the HHS where the river is cutting through a compound delta that was formed during several late Holocene highstands, all of which reached elevations between 1245 and 1255 m. Downstream from the delta, incision is much less, ranging from 3 m directly below the delta to a few decimeters adjacent to the lake where the newly formed channel has had little time to incise into the former lake bed of the rapidly receding lake. Results of this study suggest that the amount and rate of incision caused by falling base level is partly dependent on far-field slope and the distance that head cuts migrate upstream is dependent on material properties of the channel bed, where hard features such as bedrock or anthropogenic structures can arrest migration.