IMPACTS OF A LARGE RESERVOIR: LAKE MCCONAUGHY, NEBRASKA, USA

Lake McConaughy, a 60-year-old reservoir covering over 14,000 ha and having 169 km of shoreline, is an important example of the impacts of large-scale engineering on geomorphology, environmental geology, and human society. Built as an irrigation project for the equivalent of about $510 million, Lake McConaughy is now a major recreation center with surrounding properties valued at about $70 million. The lake receives more than 600,000 visitors yearly, a figure equivalent to nearly 70 times the permanent population of the surrounding county and 35% of Nebraska's total population. These visitors contribute millions of dollars to the local economy, while power generation and irrigation water values have even greater regional economic significance.

The financial impact of Lake McConaughy was at least partially anticipated by its planners, but its ultimate environmental impact was unpredictable. Waves and mass wasting (undercutting, block toppling) at high lake stands have produced wave-cut bedrock terraces, cliffs, stacks, caves, and tombolos at the high-water line of the rocky southeastern shore of the lake. Waves are strong enough to have endangered the integrity of the lake's loess-cored dam at least once (1972), and 266 m of shoreline retreat has occurred at one site. Wave erosion, gullying, and eolian reworking of sediments now endanger high-value lakeside properties. Simultaneously, fluvial deposition is filling the reservoir from the west: a sandy delta of the North Platte River has prograded at least 4.1 km since 1952, a year after peak storage was first reached. This delta began as a group of relict braid bars (separated by six large channels) partially drowned by rising pool level. By 1978, two large channels dominated, and by 1993, a single, large channel dominated the delta at low pool level. Fan deltas (0.7-1.8 km wide), built around pre-existing valley-side alluvial fans visible in pre-reservoir airphotos, have also prograded from the far southwestern shore. Furthermore, long-term monitoring wells near the lake show elevation of water tables (by up to 19 m) beginning in 1942, within a year after dam completion.

The impacts of the lake have achieved sufficient magnitude in the regional economy and environment to be public concerns. Thus, the lake's value as a case study should be realized.