Hydrology | DRAMS | Four Corners Water Center

Hydrology

Hydrology is the study of surface and ground water. Due to the diversion, then damming of the Dolores River, its hydrology has changed significantly in the last hundred years of human influence. Currently, most of the water that would naturally flow into the river from spring snowmelt is stored in McPhee Dam. Periodic release of water from McPhee Reservoir differs greatly from a natural system, and creates high flow events that have marked effects on the hydrology of the Dolores.

Annual peak flows are important for sediment scouring, riparian vegetation growth, native fish populations, and maintaining balance within the river channel. Dam-controlled releases cause a decrease in annual peak flows, which creates an imbalance in the processes of the Dolores River.

As seen in the image above, the rate of water flowing through the lower Dolores River decreased significantly after the construction of McPhee Dam, with maximum flows dropping by almost 3000 cfs. The pre-dam maximum flow in 1973 reached over 8000 cfs, while the post-dam maximum flow in 2005 barely exceeded 5000 cfs. We see this decreasing trend in flow rates because most of the snowmelt runoff that would naturally flow downstream is held in McPhee Reservoir, and McPhee Dam controls the amount of water annually released into the Dolores River. During years of low annual precipitation, the dam may release very little water downstream, as seen in 2020, where there was very little flow in the lower Dolores River. Even when there is enough water to allow a large-scale dam release (as in 2023), the flow rate is sometimes not high enough to maintain some of the natural processes of the river.

McPhee Dam Release and Surface Water

By putting various flow rates into perspective, we can better understand the impact that dam releases of varying magnitudes have on the Dolores River and its surrounding environment.

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 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 are 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.

Groundwater

By putting various flow rates into perspective, we can better understand the impact that dam releases of varying magnitudes have on the Dolores River and its surrounding environment.

Groundwater is water that exists in between rocks and soil grains below the Earth’s surface. Groundwater accounts for approximately 40% of the public water supply in the U.S. (Source: USGS) and is an important ecological resource for stream systems. In between high flow events (e.g., monsoon storms or dam releases), groundwater keeps water in streams.

Have you ever wondered why rivers have water in them even when it isn’t actively raining? The primary contributor to the flow during this time, is groundwater; water moves from the ground, through the porous walls of the channel, and into the stream. In addition to keeping water flowing in rivers, groundwater also functions as a sponge, storing water for riparian vegetation during dry seasons. Groundwater recharge is important for ecological communities and the species that rely on having water in the stream channel.

When a stream is at high flow, surface water from the channel will percolate down into the surrounding aquifer, recharging the groundwater. When a stream is at low flow, the groundwater from the aquifer contributes to the baseflow of the stream.

Big Gypsum Valley Groundwater Monitoring Data

The fluctuations in stream discharge are shown below as a blue dotted line.

Notice that there are high flows in August and September. These peaks in streamflow are associated with monsoon storm events. Monsoon storms are usually high magnitude, short duration events, which create high magnitude, short duration flows.

During high discharge events, groundwater levels increase as water is pushed through the pores of the stream channel, into the banks, and eventually travels underground away from the stream. The greatest groundwater recharge occurs near the stream (Well 1, red), but monitoring wells further from the channel (Wells 2 and 3) still “feel” the effects.

It isn’t the highest magnitude flows (flows with the greatest cfs) that most effectively recharge groundwater, but rather moderately-high to high flow events that occur over a longer period of time. Notice the first significant high flow event (discharge) that occurs at the beginning of August. Although this event isn’t the event with the greatest flow (highest magnitude), it occurs over a longer period of time and is more effective at recharging groundwater than flows over double its size that occur over a shorter period of time. This information is important to keep in mind as we think about how water is released from McPhee Reservoir and the impact on the groundwater downstream.

For more information about groundwater recharge along the Dolores River, read Dam Release And Monsoon Controlled Recharge and Drawdown of Riparian Aquifers, Dolores River, Colorado.