McPhee Dam Release and Surface Water
The figure below is a hydrograph of comparing Dolores River annual flow rates from 2017-2018 and from 2018-2919 at three different USGS gauge stations. The Dolores, CO (in red) gage station is located above McPhee Dam, while the Slick Rock, CO and the Near Bedrock, CO (both in green), are located below the dam.
Dolores Discharge Without Dam Release
On the topmost graph, the peak flow (between 2018-03-15 and 2018-06-01) is seen above the dam and not seen below the dam because the discharge recorded by the gage stations above the dam is filling McPhee reservoir rather than continuing to flow downstream to the next gauge station
Peak flows occur in the spring when rising temperatures increase runoff from snowmelt. Low flow (2018-01-01 to 2018-03-15) occurs when there is no input into the river other than groundwater.
Notice the brief spikes of high flow the in the late summer. Those are flash floods related to summer thunderstorms. These types of events contribute some water and significant amounts of sediment to the river below the dam.
Dolores Discharge With Dam Releases
The bottom graph shows annual flow rates at and below the dam one year later, from 2018 to 2019 This was a high flow year with significant snowmelt, which filled McPhee Reservoir.
Notice how the shape of the flow at Dolores, above the dam, is very similar to the shape of the flow at Slick Rock, below the dam. Because of high flow above the dam, the dam operators were able to release water. This means that there is similar flow trends further down the river at Slick Rock, it just takes time for the river to see it. There was about a week delay of flow from above the dam to below the dam near Bedrock.