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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.
The charts below dive into the interaction between the discharge of the Dolores and the McPhee dam releases.
Each blue triangle on the map below represents a study site where hydrology data were collected for previous Dolores River studies.
On the map below, each cross () indicates the location of a USGS stream gage site. The green crosses () indicate the locations of stream gages downstream from the McPhee Dam. The red cross () is the Dolores, CO stream gage, which is located upstream of the McPhee Dam. These color-coded stream gage symbols are consistent with the graphs below that compare streamflow in dam release water years to non-dam release water years.
Big Gypsum Valley is the only groundwater monitoring site within the scope of the DRAMS project. However, there is continual groundwater monitoring at two other sites upstream of Big Gypsum Valley that are maintained by Dr. Clutter and her team.
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.
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.
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.
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.
Fort Lewis College professors Dr. Melissa Clutter, Dr. Gary Gianniny, and Dr. Cynthia Dott have conducted seasonal groundwater monitoring along the Dolores since 2010.
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 click on the link here .