STRATIGRAPHIC EVOLUTION OF LAS VEGAS BAY, LAKE MEAD, NEVADA: 1935 – 2002, PART 2 – THE SUBAERIAL PORTION

A delta in Las Vegas Bay, Lake Mead, NV underwent 1.5 km of normal regression during an ~17 year period of high lake level and elevated sediment supply, followed by a recent 1.4 km forced regression as a result of a 20 m drawdown in the water level of Lake Mead over a three year period. Two bathymetry surveys conducted during the recent drawdown in lake level illustrate that the majority of sediment accumulation has been confined proximal to the Las Vegas Wash Delta.

The current drawdown in lake level has resulted in fluvial incision of abandoned high lake level deposits and provides an outcrop record of the Las Vegas Wash Delta stratigraphy. Historic shoreline mapping in conjunction with detailed photomosaic mapping of recently incised fluvial-deltaic and shallow water deposits within the Las Vegas Wash illustrate the effects changes in sediment supply, fluvial discharge, and lake level have had on controlling the historic positions of shorelines and the resulting stratigraphy.

Fluvial terraces developed on post-impoundment deltaic deposits are common throughout the lower Las Vegas Wash. It was initially believed that these terraces recorded episodic stillstands in lake level during an overall drawdown period. However, in light of recent experimental studies and the photomosaic mapping in Las Vegas Wash an alternative hypothesis is that these fluvial terraces developed as a result of an autoincision process during a continual drawdown in lake level. Limited GPR data reveal complex geometries within the internal stratigraphy of the Las Vegas Wash Deltaic deposits.