Axial river evolution in response to half-graben faulting: Carson River, Nevada, USA

The Holocene history of faulting and river-channel movement for the Carson Valley, Nevada, allows the tectonic-alluvial interactions of an active half-graben to be studied. Topographic surveys, maps, cores, radiocarbon dates, and paleohydrology are used to study the temporal and spatial patterns of Carson River channel change. The influence of faulting and associated ground tilting on channel movement is determined by comparing the pattern and timing of river movements with the published event history of the basin-bounding fault. Inverse modeling of the fault history, assuming rigid block motion, suggests that surface gradients perpendicular to the channel and basin axis have progressively increased through the Holocene, from an initially horizontal surface. The greater part of this movement occurred during and immediately after faulting. Channel avulsion in response to this increased tectonic ground tilting appears to be the most important factor controlling alluvial architecture, although intrinsic fluvial processes and regional climate change may have triggered individual avulsions, Channel belts avulsed towards the fault (onlap) after each tectonic event; during periods of tectonic quiescence they migrated away from the fault (offlap), taking the path of maximum overall gradient. Lag times between faulting and channel avulsion are very short (instantaneous to hundreds of years). The observed Holocene history of the Carson River is consistent with positions of channel belts predicted by half-graben stratigraphic models, and therefore this study provides the first confirmation that the underlying alluvial-tectonic assumptions of these models are broadly correct.