Spring has been good to Lake McConaughy this year, both in terms of inflows and outflows: quite a bit of the former and not so much of the latter. Several factors combined to bring about these circumstances.
First, snowpack in both the North Platte River Basin and the South Platte River Basin was well above median (normal) levels. When the spring melt began, the water began flowing into storage reservoirs. Despite the fact that the federal reservoirs on the North Platte River in Wyoming were well below normal in terms of carry-over storage, the mere fact that storage supplies were being replenished was good news.
Entering May, there was little cause for optimism. During a period when inflows to Lake McConaughy historically begin to rise, inflows were relatively flat and hovering around 60% of normal. However, shortly after the U.S. Bureau of Reclamation began moving water from its larger upstream reservoirs down to Glendo Reservoir in preparation for releases to irrigation canals in eastern Wyoming and western Nebraska, the lower basin was hit with a series of heavy snowfall and precipitation events. The additional inflow pushed Glendo Reservoir into its flood pool and water was released down the North Platte River to make room for the extra water entering Glendo.
As a result, Lake McConaughy benefited from an unexpected spike in inflows that lasted from mid-May until the middle of June. Central’s main storage reservoir – thought to have peaked in early May – rose to a second peak near the end of June before inflows diminished, falling back below historic median flow rates.
At the same time, high flows in the South Platte River resulting from heavy snowmelt runoff and precipitation from spring thunderstorms entered Nebraska. For a period of several weeks, Central could hold water in Lake McConaughy and divert excess South Platte water into the Supply Canal to help fill the irrigation canals. Water in excess of amounts that could be diverted continued on down the Platte River, exceeding the U.S. Fish and Wildlife Service’s target flows for wildlife habitat purposes and allowing Central to divert excess flows into Elwood Reservoir for groundwater recharge purposes in both the Platte and Republican River basins. As of June 24, Elwood Reservoir’s elevation had gone up by more than eight feet.