2023 Dam Release

Dolores River Home Hydrology Riparian Vegetation Geomorphology DRAMS Team

Introduction

Due to a large seasonal snowpack, the McPhee Reservoir received large amounts of water during the 2023 water year. To accommodate the large influx of water in the reservoir, a “dam release” occurred where a controlled amount of water from the reservoir was released into the downstream sections of the Dolores River. On May 4th, 2023, McPhee Reservoir began releasing water which marked the start of the 2023 dam release; the release extended for two months before officially ending on July 1st, 2023.

Slick Rock hydrograph data and corresponding images from two different dates at the Gypsum Valley Boat Launch. The images compare the flooding extent and vegetation growth while the river was experiencing high (4220 cts) and lou images compare the flooding extent and vegetation growth while the river was experiencing high (4220 cfs) and lon discharge (~0 cfs). Infrared light can detect areas with high vegetation (red) and areas with water (blue). The hydrograph was derived from data collected by the USGS near Slick Rock, Colorado
Photo credit: Jon Harvey
Data: https://lpdaac.usgs.gov/data/get-started-data/workflow-examples/ — Slick Rock hydrograph data and corresponding images from two different dates at the Gypsum Valley Boat Launch. The images compare the flooding extent and vegetation growth while the river was experiencing high (4220 cts) and lou images compare the flooding extent and vegetation growth while the river was experiencing high (4220 cfs) and lon discharge (~0 cfs). Infrared light can detect areas with high vegetation (red) and areas with water (blue). The hydrograph was derived from data collected by the USGS near Slick Rock, Colorado Photo credit: Jon Harvey Data: https://lpdaac.usgs.gov/data/get-started-data/workflow-examples/

What is a Dam Release?

The Dolores River upstream of McPhee Reservoir naturally experiences large changes in interannual and seasonal flows resulting from winter precipitation, spring melt, and summer monsoons. However, since the construction of the McPhee Reservoir, the flow regime downstream of McPhee is far different from the natural flow regime.

A dam release occurs when water is released from a dam above the typical amount. During these controlled releases, a specific discharge (in cubic feet per second (cfs)) is released over a duration of time which simulates a high-flow event. or to the 2023 dam release, the last large-scale release from McPhee occurred in 2019, due to the large snowpack that formed between 2018 to 2019. The high flows that occurred in 2019 are expanded upon more on the /dolores-river/hydrologyHydrology page which includes hydrographs that display the releases that occurred from 2018 to 2019 and make comparisons to prior low flow years.

This animation displays images of the Dolores River near Slick Rock, Colorado over the course of the 2023 Dam release and compares them to the amount of discharge displayed on the associated hydrograph.

Why Dam Releases Matter

The construction of the McPhee Reservoir influences the biology, hydrology, and geomorphology downstream of the Dolores River and can create long-term impacts on each of these disciplines. These downstream impacts are not only affected by the presence of the reservoir itself but also by dam operations. With the emplacement of the McPhee Dam, the natural flow regime of which water is released downstream from the dam is heavily altered, and releases are determined by dam management practices.

Riparian Vegetation

In the past, river regulations have focused on river quality and meeting minimum flow requirements needed to maintain river ecosystems and aquatic species. However, the natural flow regime for a river ecosystem has been increasingly recognized as a critical part of sustaining native biodiversity and ecosystem integrity.

The flow regimes along the Dolores River influence “riparian corridors’- unique plant communities found next to bodies of water that are important indicators of river health. The Dolores River has provided a unique opportunity to observe the nature of flow regiments before and after the construction of the McPhee Dam by monitoring changes in riparian corridor vegetation over time. The /dolores-river/riparian-vegetationriparian vegetation page expands more on the impacts of the construction of McPhee Dam on the flow regime of the Dolores River and the correlation flow regime has on river health.

Groundwater

The amount of water released from the McPhee Reservoir also heavily impacts the groundwater along the Dolores River downstream. Groundwater provides another important ecological resource for stream systems and is responsible for delivering water to the Dolores River and surrounding streams. Dam releases provide high-flow events that occur over longer periods of time, which are most effective in recharging the groundwater.

The Hydrology page expands more on factors that influence groundwater levels and provides visual graphics that support how high discharge events further affect groundwater levels surrounding the Dolores River.

Geomorphology

The geomorphology of the Dolores River is also impacted by the amount of water released downstream. The shape and features of rivers are influenced by the interaction of high and low water flow, sediment, and the presence of vegetation. During high-flow events, the channels along the river erode and deposit sediment downstream.

This process results in channel migration which creates various features that shape river systems which are important to providing native fish species with the appropriate habitat conditions. Low-flow conditions have deprived the Dolores River of these aspects and features which are further investigated on the /dolores-river/geomorphologygeomorphology page.

For 2023 Flood Page, Vegetation Patterns

Summary Findings of the DRAMS Vegetation Monitoring Effort:

Patterns of vegetation change across the 5 sites and over the 5-year monitoring period exhibited some significant trends. These trends were most clearly summarized by exploring each of the four major geomorphic zones identified in the Riparian Vegetation Summary (link to veg summary page).

During the study period of 2021-2025, only one year (2023) experienced high summer flows due to dam release of snowmelt waters, and the last prior year to have a large summer dam release was 2019. However, post-dam “floods” are much lower in magnitude than pre-dam high flows (link to hydrology page – historic Dolores river flows chart – hydrograph). Two years during the study period had brief spikes in flow due to large monsoonal rain events (2022 and 2024 especially). Three years (2020 – the year preceding our study – 2021 and 2025) had minimal or no high flows of any kind (see Figure 1). This variability in flows from one year to the next allowed us to see first-hand what impacts low and high water could have on the vegetation and on the river channel, and thus on fish habitat.

We can summarize our key findings for the different geomorphic zones into two broad categories, based on flow dynamics: the negative effects of multiple low-flow years, versus the benefits of the 2023 high flow flood event.

Persistent Low Flows have negative impacts on wildlife habitat via vegetation change, by the following three avenues:

Negative Impact 1- Bar colonization: up to 40% increase in vegetation cover in 4 years

Open cobble bars become increasingly covered with vegetation during low flow periods, which makes them less suitable for fish spawning habitat (Figure 2A and 2B).

Similar changes were observed at multiple sites (See Figure 3 for an example):
Number of woody stems (mostly willow) nearly doubled from 2022 to 2025 (from about 5-10 stems/m² in 2022 to ~20 stems/m² in 2025).
Cover by non-woody plants (sedges, grasses, etc.) on bars increased by between 15% to over 40% from 2022 to 2025, causing a decrease in bare cobble area by 20-35%.

Negative Impact 2 - Channel simplification: steady increase in willow stem density on open banks

Willows colonize open bank habitat during low flow periods, and gain a stable “toe-hold” in these areas (Figure 3, Figure 4)

The willow colonization leads to channel narrowing and simplification via vegetation encroachment, which decreases habitat diversity for native fish (Figure 5). (link to “willow zone” on riparian veg summary page)

Negative Impact 3 - Floodplain transition: after several dry years, upper floodplain vegetation cover (at a site with cottonwoods) included ~3% cottonwood cover and over 25% upland shrubs

Multiple years of low flows and no peak spring/summer discharge are detrimental to riparian species and wildlife habitat. The lack of water table recharge leads to dieback of valuable cottonwood trees (Figure 6).

Under these drier conditions, the floodplain transitions from a moist habitat to a drier habitat, and is increasingly invaded by upland shrub species which displace riparian shrubs. At Big Gypsum, there was an increase from 18% to 28% cover by these species from 2021 to 2022 (Figure 7). Riparian plant species support much higher wildlife diversity than the drier uplands.
(link to “upland scrub zone” on riparian veg summary page)

As shown above, we found that significant challenges arise during periods of drought and very low flows in the Dolores River, via the three avenues of change described.

A single year with Prolonged High Flows such as occurred in 2023, can have immediate benefits for the channel and the floodplain by driving changes in the vegetation. Again, there were three main avenues for change:

High Flow Positive Impact 1 - Bar/cobble scouring: flood flows can temporarily decrease plant cover on bars by up to 40% in a single year

High flow events inundate and scour cobble bars and banks, slowing down colonization by vegetation. After a single year, the impact appears to be highest for herbaceous (non-woody) vegetation (Figure 8).

2023 flood flows drove herb cover down by about 40%, but given the rapid rebound in cover that occurred during the following low flow year it appears that repeated scouring is required to keep bars open.
Since open cobble bars are important fish spawning habitat, frequent high flow events are needed to slow vegetation expansion and remove silt.

High Flow Positive Impact 2 - Floodplain water table recharge: willow and cottonwood cover increased by nearly 10% to over 25% after 2023 floods at some sites

High flow events re-charge and raise the water table in the floodplain, which has immediate benefits for riparian willows and cottonwoods (Figure 9).

These riparian shrub/tree species require close contact between their roots and the surface of the water table. During prolonged drought and low river flows, the water table falls too deeply for them to reach and dieback occurs (see Figure 6) (Dott et al. 2016 – pdf in folder).
Willow and cottonwood cover increased over the course of the study, and especially after the high flows of 2023. This was evident from our data, and also from repeat photo examples (Figure 10).

High Flow Positive Impact 3 - Inundation of upland shrubs: floodplain cover by dryland shrubs was cut in half by 2023 flooding, and continued to drop in 2024

High flow events “re-claim” the upper floodplain from take-over by upland shrubs like sagebrush, rabbitbrush and saltbush. While dry years did lead to an increase in upland shrub cover (Figure 7), the 2023 floods caused a significant decline in shrub abundance: from nearly 4 stems/m² to less than 2 stems/m²; and from nearly 30% total cover to less than 15%. Unlike on cobble bars (Figure 8), the decline in upland shrubs persisted for multiple years after the initial flood year (Figures 11 & 12).

Summary of Positive Impacts of High Flows

While prolonged drought leads to low flow years on the Dolores River, it was encouraging to learn that even occasional high flow events such as what occurred in 2023 can still have rapid and beneficial impacts on riparian vegetation and wildlife habitat. These positive impacts include scouring of low-lying cobble bars to keep them more open for fish habitat; re-charging the floodplain water table which supports cottonwood and willow growth; and inundating or drowning the upland shrubs that have moved onto the upper floodplain areas that could support more riparian shrub species.

Management Implications of the DRAMS Vegetation Monitoring Work

Willows – A Mixed Blessing:

One challenge managers face on the Dolores River is driven by willow colonization, which has contributed to channel narrowing throughout the river corridor. Coyote willow (Salix exigua) is the dominant species, and unlike tamarisk is a native member of our riparian communities in this region. It provides important cover and forage for wildlife (birds, beaver, etc.), and helps limit erosion. However, as we have seen, it also impinges on river banks, narrows the channel, and impacts fish habitat as well as river recreation.

Based on our results so far, it appears that willows benefit from both low flow and high flow conditions:

Persistent low flows allow willow to colonize low bars on the river’s banks which are under water during high flows, and which once served as important fish spawning areas and contributed to habitat complexity of the river channel.
High flows that are prolonged over multiple weeks are extremely beneficial to all life in the river corridor, and are critical to fish. These flows also recharge the water table and allow for survival of valuable riparian species on the floodplain such as cottonwoods. Willows also increase in cover or density following high recharging flows such as these, making their bank-fringing cover even more dense.
In a system where more frequent high flow events could occur, this dual nature of willows would be balanced and some banks and bars would remain open or be re-opened by periodic flood events. But current conditions on the Dolores River do not allow for that kind of balance in flow events.
Thus, to best manage for habitat complexity and diversity, and to support native fish, some active vegetation management will be required. One place where we have discussed implementing active willow removal would be at the upstream ends of old side channels, so that higher water could flow through those areas and allow fish access during periods of moderate to higher discharge (Figure 13).

2023 Flood Findings – Hydrology

During the 2023 McPhee Dam release, the Dolores River experienced a high magnitude flooding event where water overflowed from the river channel onto the surrounding floodplain. During a flooding event like this, water infiltrates not only from the riverbanks but also across the flooded land surface, ultimately recharging underlying aquifers.

The magnitude and duration of these high flow events are key to effectively recharging groundwater aquifers adjacent to stream systems. When high magnitude, long duration floods occur (like during a large dam release), water is absorbed into the surrounding riparian aquifer. After these events, it takes longer for water to drain out of the system, especially when compared to short-duration monsoon storms (even if the monsoon storm causes a high magnitude flood event).

Data collected from the Big Gypsum Valley Groundwater Wells during the 2023 McPhee Dam release shows the impact of prolonged duration and high magnitude flood events on groundwater recharge near the river channel. Water not only infiltrated through the banks of the main channel but also infiltrated through the surface during inundation.

The locations within the pointbar that were inundated for the longest duration were two mid-bar channels. Channel 1 has an average width of 40 ft and maximum depth of ~3 ft and channel 2 has an average width of 110 ft and average depth of ~5 ft. Channel 1 was inundated for over a month when the Dolores River was greater than ~1300-2000 cfs and Channel 2 was inundated for several weeks when the Dolores River was greater than ~2500-2980 cfs.

The cross-cutting point bar channels indicate a path of least resistance across the point bar and are carved by higher flows during periods of increased discharge.

When looking at the riparian vegetation present on the point bar, coyote willows (Salix Exigua) populate the banks of the main river channel but are also found on either side of the two cross-cutting point bar channels.

The dam-controlled flow regime of the Dolores creates favorable growth conditions for these willows. The presence of willows on either side of the cross-cutting channels encourages further channelization through the point bar since willows’ root system traps sediment, creating natural levees that will further isolate these channels from the surrounding point bar.

The presence of coyote willows on the sides of these cross-cutting channels indicates a higher water table in the area since their roots need to be close to water to ensure their survival.

From groundwater well data, it was determined that prolonged inundation did contribute to noticeable groundwater recharge in the riparian aquifer. This recharge occurred due to the duration and magnitude of the dam release.

Because the release was so much larger than past years, it almost fully inundated the point bar at its peak and inundated cross-cutting channels for prolonged periods of time, allowing recharge to occur from the surfaces around the river.

2023 Dam Release | DRAMS | Four Corners Water Center