The northwest and west inlet streams contribute the largest quantities of water to Mirror Lake. On average, they contribute about 63 percent of the water input to the lake. The stream discharges from these two largest watersheds of the lake are measured using Parshall flumes, which are described at Stop 2. However, at these stream gauges, weir plates are not installed at times of low flow as at the outlet because these flumes are smaller and can measure low stream flows relatively accurately.

The flow characteristics of the northwest and west inlet streams are typical of steep mountain watersheds; that is, runoff is rapid following rain resulting in large quantities of water being discharged in a short period of time, and a rapid decline of discharge following peak flows. Although the hydrographs of these two streams are similar, they do have some differences.

	Streamflow hydrographs on August 11, 2003 for northwest inlet stream (left) and west inlet stream (right)

The recession slopes; that is, the slope of the curves following peak discharges are a bit flatter for the Northwest (NW) stream than they are for the West (W) stream. The reasons for this are related to the watershed characteristics and their geology. For example, the shapes of the watersheds are quite different. Watershed NW is oblate in form, whereas watershed W is long and linear, more or less wrapping around the west side of watershed W.

The geology of the watersheds is also quite different, especially with respect to the glacial deposits that lie within them. There is a greater volume of glacial deposits in watershed NW than there is in watershed W, and the greatest volume of the deposits in watershed NW occurs in the lower part of the watershed. This greater volume of glacial deposits in the lower part of watershed NW results in more ground water being stored here, and the slow release of this ground water to the stream is the reason the recession slopes are flatter in watershed NW compared to watershed W.

Another geologic factor that might account for some of the difference in flow between these two inlet streams to Mirror Lake is the distribution of different types of glacial deposits in the watersheds. Watershed W has a number of terraces at different elevations in the watershed, and the geology of the terraces also differs somewhat. Some of them have sand and gravel overlying till and others are composed entirely of sand and gravel. Stream W passes between two of the terraces that consist of sand and gravel (FS1 and FS2), and in this reach of Stream W it loses water by seepage to ground water. As a result, less water reaches the flume where the discharge is measured than would have if some of the stream flow had not been lost to ground water.