t ?.. ~" , 177 ELSEVIER Sedimentary Geology92 (1994) 7-16 SEDIMENTARY GEOLOGY ExpresSed Historical adjustments by Walker River to lake-level fall over a tectonically tilted half-graben floor, Walker Lake Basin, Nevada Terence C. Blair a, John G. McPherson b,c a Blair and Associates, 1949 Hardscrabble Place, Boulder, Colorado 80303, USA b Mobil Exploration and Producing Australia Pty Ltd, P.O. Box 4507, Melbourne 3001, Australia c Mobil Research and Development Corporation, P.O. Box 650232, Dallas, Texas 75265, USA Received March 25, 1994;revised version accepted April 26, 1994 Abstract 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 asymmetric 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 Walker Lake has withdrawn from the northern basin sector since 1882 due to a 45 m drop in lake level caused by human diversion of the river upstream.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. 1. Introduction The dissimilar reaction rate to neotectonic de- formation between marginal alluvial fans and ax- ial fluvial, lacustrine, or marine environments in half-graben extensional basins was appreciated decades ago (e.g., Russell, 1883, 1885; Myers and Hamilton, 1964; Christensen, 1966; Hunt and Mabey, 1966). More recent studies have ad- dressed specific sedimentologic and stratigraphic 0037-0738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0037-0738(94)00058-3