HISTORICAL ADJUSTMENTS BY WALKER RIVER TO LAKE-LEVEL FALL OVER A TECTONICALLY TILTED HALF-GRABEN FLOOR, WALKER LAKE BASIN, NEVADA

Historical records of lake and river adjustments in the tectonically active, north-south elongated, Walker Lake extensional basin of west-central Nevada provide important insight to the style and rate that rivers react to tectonic tilt in half grabens. The northern part of this basin, historically containing Walker Lake, is now occupied by the south-flowing Walker River. Walker Lake, presently restricted to the central basin sector, is a perennial water body 30 m deep sustained by, and forming the terminus of, Walker River. Both the river and lake have a strongly assymmetric distribution, located preferentially near the active Walker Lake fault bounding the western basin margin. Walker Lake has withdrawn from the northern basin sector since 1882 due to a 45 m drop in lake level caused by human diversion of the river upstream. This withdrawal has forced Walker River to incrementally lengthen, and to sequentially reposition its delta along the retreating northern lake margin. Twelve deltas have been deposited since 1882 in response to these changes. The initiation of eight of the twelve post-1882 deltas was associated with channel avulsion, and four with channel lengthening. Ten of the twelve channel and delta relocation events entailed significant westward lateral movements toward the Walker Lake fault, illustrating the dominating influence of the basin-floor tectonic tilt on river adjustments. The two eastward shifts were triggered intrinsically by avulsions resulting from erosion of the outer bank of channel meanders. The three phases of progressive westward shifting of the river towards the Walker Lake fault occurred over periods of 18, 6, and 16 years, respectively, documenting the rapid response time of Walker River to basin-floor tectonic tilt despite the countering effects of the intrinsic avulsions, and hindrances to lateral migration caused by the muddy substrate of the former lake bottom deposits within which the river is incised.