Riparian Vegetation Response to Flow Modification: Water Management Impacts in Canyonlands National Park – National Park Service

Desert Oases: Rivers in the Intermountain West

Rivers are vital, but limited, resources in the semiarid and arid intermountain west for people and for the ecosystems- wildlife and habitats- that depend on them. Local ecosystems evolved based on seasonal water replenishment by early summer snowmelt when pulses of water would flush channels and inundate lowlands, redistributing sediment, nutrients, and water along riverbanks. This resulted in the creation and continued maintenance of a complex river system and diverse riparian corridor. These corridors are zones of vegetation that provide several ecosystem services including helping to maintain water quality, stabilizing banks, and providing habitat that results in biodiversity hotspots and recreational and scenic opportunities for people.

These waterways also attracted human settlements and were key to the development of the western United States. Western expansion brought increased demands for water and electricity, necessitating water management and water development projects such as the Flaming Gorge and Blue Mesa Dams that were installed in the Upper Colorado River Basin in the 1960s. However, the reliable, year-round water supply and hydroelectric power they provided came at a cost to the rivers themselves as changes in streamflow (such as smaller peak flows and higher base flows) resulted in vegetation encroachment, channel narrowing, and the simplification of river morphology including the disconnection of the river from its floodplain.

Water Management in Action: Finding the Balance

In an effort to reduce the negative impacts the flow changes caused in aquatic and riparian ecosystems downstream of the Flaming Gorge and Blue Mesa Dams, dam managers modified reservoir releases in 1992 to simulate more natural flow patterns by managing for lower base flows and higher peak flows. Additional modifications occurred in 1995 to protect endangered fish under the Endangered Species Act, and for a water right put in place on the Black Canyon of the Gunnison National Park in 2008. These effectively resulted in three different hydrological periods during the 20^th and early 21^st centuries:

• Pre-dam flow: 1940-1966 (before the dams were closed and operating)
• Post-dam flow: 1966-1992 (Colorado River) and 1963-1992 (Green River) (dam flows were operated strictly for hydropower)
• Post-dam environmental flow: 1992-2022 (flows were modified for habitat and endangered species conservation)

Our Study

We wanted to understand how the initial dam installations and later modified dam management affected riparian vegetation downstream of the Flaming Gorge and Blue Mesa Dams in Canyonlands National Park.

Methods

Using available aerial imagery from 1940 to 2022 we examined vegetated area (defined as having >50% vegetation cover) along three sections (94 mi (152km)) of the Colorado and Green Rivers within Canyonlands National Park to identify changes in riparian vegetation response over 80 years of dam management for the Colorado and Green Rivers and to assess the rate of changes in total vegetation resulting from long-term channel narrowing during the 30 years that the dams were operating with altered (environmental) flows (1992-2022). Our study focused on three river sections in the Upper Colorado River Basin and included both the Green and Colorado Rivers above their confluence and the Colorado River below the confluence (called the Cataract Canyon reach). This represents the cumulative total of the seminatural flow of these rivers before entering Lake Powell (3 mi (5km)) downstream of our study location).

Desert Oases: Rivers in the Intermountain West

Rivers are vital, but limited, resources in the semiarid and arid intermountain west for people and for the ecosystems- wildlife and habitats- that depend on them. Local ecosystems evolved based on seasonal water replenishment by early summer snowmelt when pulses of water would flush channels and inundate lowlands, redistributing sediment, nutrients, and water along riverbanks. This resulted in the creation and continued maintenance of a complex river system and diverse riparian corridor. These corridors are zones of vegetation that provide several ecosystem services including helping to maintain water quality, stabilizing banks, and providing habitat that results in biodiversity hotspots and recreational and scenic opportunities for people.

These waterways also attracted human settlements and were key to the development of the western United States. Western expansion brought increased demands for water and electricity, necessitating water management and water development projects such as the Flaming Gorge and Blue Mesa Dams that were installed in the Upper Colorado River Basin in the 1960s. However, the reliable, year-round water supply and hydroelectric power they provided came at a cost to the rivers themselves as changes in streamflow (such as smaller peak flows and higher base flows) resulted in vegetation encroachment, channel narrowing, and the simplification of river morphology including the disconnection of the river from its floodplain.

Water Management in Action: Finding the Balance

In an effort to reduce the negative impacts the flow changes caused in aquatic and riparian ecosystems downstream of the Flaming Gorge and Blue Mesa Dams, dam managers modified reservoir releases in 1992 to simulate more natural flow patterns by managing for lower base flows and higher peak flows. Additional modifications occurred in 1995 to protect endangered fish under the Endangered Species Act, and for a water right put in place on the Black Canyon of the Gunnison National Park in 2008. These effectively resulted in three different hydrological periods during the 20^th and early 21^st centuries:

Pre-dam flow: 1940-1966 (before the dams were closed and operating)
Post-dam flow: 1966-1992 (Colorado River) and 1963-1992 (Green River) (dam flows were operated strictly for hydropower)
Post-dam environmental flow: 1992-2022 (flows were modified for habitat and endangered species conservation)

Our Study

We wanted to understand how the initial dam installations and later modified dam management affected riparian vegetation downstream of the Flaming Gorge and Blue Mesa Dams in Canyonlands National Park.

Methods

Using available aerial imagery from 1940 to 2022 we examined vegetated area (defined as having >50% vegetation cover) along three sections (94 mi (152km)) of the Colorado and Green Rivers within Canyonlands National Park to identify changes in riparian vegetation response over 80 years of dam management for the Colorado and Green Rivers and to assess the rate of changes in total vegetation resulting from long-term channel narrowing during the 30 years that the dams were operating with altered (environmental) flows (1992-2022). Our study focused on three river sections in the Upper Colorado River Basin and included both the Green and Colorado Rivers above their confluence and the Colorado River below the confluence (called the Cataract Canyon reach). This represents the cumulative total of the seminatural flow of these rivers before entering Lake Powell (3 mi (5km)) downstream of our study location).

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Source: nps.gov