Mirror Lake loses water by way of surface outflow, evaporation, and seepage to ground water. Knowing the amount of water that a lake loses is important in understanding the function of lakes because it allows us to determine the length of time water remains in a lake; that is, its residence time. The losses of water by surface outflow are greatest when the lake flows over the top of the dam, usually during spring snowmelt and during heavy rainfall. When the lake level is lower than the top of the dam, the lake still loses some water by seepage between the wooden boards that comprise the dam.

Outlet flume downstream of dam

To measure the total amount of surface outflow, both the flow over the dam and the seepage between the boards, we installed a flume a few meters downstream of the dam.

A flume is a structure specifically designed to measure the discharge of streams. It is built in the streambed and is designed to allow the stream water to flow through the structure. By knowing the level (stage) of the water in the inlet section, stream discharge can be calculated using special formulas developed for each type of flume. For the Mirror Lake studies, we chose to use Parshall flumes to measure stream discharge to and from the lake. A Parshall flume consists of three sections; the inlet section, the throat, and the outlet section. To measure the water stage in the inlet section of the flume, a stilling well is used. The stilling well is connected to the flume by way of a short pipe, which allows the water stage in the flume and in the stilling well to be exactly the same.

Flume stilling well recorder and completed chart

Similar to the lake stage gage, a recorder and float system is placed on the stilling well so the water stage in the inlet section of the flume can be continuously recorded. Once this water stage is known we can calculate the discharge through the flume.

At very low flows, Parshall flumes can be somewhat inaccurate because the small amount of water passing through the flume throat is very shallow and it does not flow uniformly across the width of the throat. To make the outlet flume more accurate at times of low flows, we modified the flume, converting it to a weir. A weir is another type of structure that is used to measure the discharge of streams. The type we use is called a v-notch weir.

Outlet flume with weir plate in place

A weir consists of a flat plate that has a notch cut into its top, in our case the notch has a 120 degree angle. Furthermore, the notch is beveled to a sharp edge. In order for a weir work properly, the weir plate must act as a dam that backs up a pool of water behind it large enough so there is no water flow velocity at the notch. In other words, the water must fall freely through the notch simply by the force of gravity.

Similar to flumes, discharge at a weir is calculated from special formulas using the stage of the water pool behind the weir plate. Therefore, when surface flow from Mirror Lake decreases to very low quantities, the weir plate is bolted to the outlet end of the flume. This causes water to back up behind the plate to where the flume itself encloses part of the storage pool. By doing this, the same stilling well and float/recorder system that is used for the flume calculation can be used for the weir calculation. When the weir plate is attached, a notation is made on the flume chart and the formula for the weir is used instead of the formula for the flume. Conversely, at the end of the low-flow season the weir plate is removed and the structure goes back to operating as a flume.