Geomorphology

Dolores River Home Hydrology Riparian Vegetation DRAMS Team

What is geomorphology and why are we looking at it?

Geomorphology is the study of the form or shape of the landscape, and how it changes through time. In the case of rivers, the focus is on the shape of the river channel and its floodplain. Changes to a river happen both naturally and as a result of human actions. For example, during extreme flood events, river channels often get wider and/or deeper, and may change their course completely. The geomorphology of a river influences fish habitat, water supply, riparian vegetation, and the recreation experience.

the DRAMS team wades in a canyon river with nets and gear, surrounded by green trees and vegetation and red cliffs.

The geomorphology of a river is shaped by the flow of water that travels down it. We keep track of the flow of a river by measuring its discharge in units of cubic feet per second, or 'cfs'. Over the time that we have been tracking discharge, there has been a major decrease in the discharge of the Dolores River due to irrigation diversions, the construction of the McPhee Dam, and climate change in the watershed. The hydrographs in the Hydrology section show the discharge of the Dolores River both upstream and downstream of McPhee Dam over the course of a typical water year. Notice how the presence of the dam stops the typical spring runoff and monsoon flood peaks from continuing downstream. In this section, we document some of the observed changes to the river channel and the floodplain that have resulted in the changing hydrology of the Dolores river below McPhee Dam.

Geomorphology Primer

Before we dive in, here are some geomorphology terms that may be helpful to have in mind, overlain on an aerial photograph of the Dolores River.

All of these features of the river are shaped and influenced by the interaction of flow (highs and lows), sediment, and riparian vegetation. During high flows, the river erodes its outer banks and deposits this sediment on bars downstream. This results in channel migration. As the channel migrates, it leaves behind old channels, which can become side channels with slower-moving water. Side channels can also form around or behind mid-channel bars and islands that might form where sediment naturally deposits, or during large flood events where new channels are carved out of the floodplain. Riparian vegetation often serves to slow down channel migration, anchor sediment on bars and islands, narrow the channel, and create complex habitat conditions.

satellite imagery of the Dolores River, with arrows pointing to deposition on bank-attached bar, erosion of cut bank, channel width, mid-channel bar, and side channels.

Observed Changes to the Corridor

In response to the dramatic changes in Hydrology and Riparian Vegetation resulting in large part from McPhee Reservoir, the geomorphology of the Dolores River has also changed.

Abundant anecdotal evidence of changes to the Dolores River from those who have known it and used it for decades includes the observation of:

The channel narrowing
Loss of side channels
Steep vegetation choked banks forming along the river
A decrease in the number of unvegetated sand and gravel bars

These trends are alarming for native fish habitats because fish are most successful when they have a variety of in-channel environments in which to live and breed. Deep pools, riffles, cobble bars, slow-moving side channels, and backwater sloughs are all examples of different channel sub-environments that together, create a system of geographically complex riverine habitat referred to as 'channel complexity'.

The research summarized below is focused on two questions about channel complexity on the Dolores River downstream of McPhee Dam:

How has channel complexity changed since the dam was built?
On a modern river, what kind of complexity can we achieve at different discharges.

The DRAMS team is documenting and measuring those and other changes to the geomorphology of the river corridor by systematically comparing aerial (from the air) photographs from decades ago to modern aerial images. These measurements are taken within DRAMS study segments:

Study Segments (upstream to downstream)

Bradfield
Ponderosa Gorge
Lower Ponderosa
Below Disappointment Creek
Slickrock Canyon
Below San Miguel

Geomorphology study segments color coded on a map of the Dolores River.

This exciting work is possible because of a series of aerial photo sets that were collected in 1981 and 1982. By carefully painstakingly georeferencing both 1980's aerial imagery and modern satellite data and comparing these images in the context of the Dolores River, we can measure and quantify the amount and speed of the geomorphic changes.

Below is a swipe-enabled map that compares the two datasets mentioned above. The dataset on the left is from 1982, while the dataset on the right is from 2019. The 2019 dataset is displayed in false color infrared to aid in the display of vegetation: the brighter the red, the more vegetation there is in an area. Use the zoom buttons to focus on different study sections and the swipe function to compare the two datasets. Keep an eye out for the areas with the most dramatic changes!

View larger map

River Features and Channel Complexity

When analyzing a river system, it is helpful to focus in on specific river features to help tell a story about the changes and trends that we observe. Although the work is ongoing, efforts by FLC Geoscience faculty and several students have already reached some important conclusions by focusing on the following features:

Unvegetated Gravel and Sand Bars: Ridges comprised of sand or gravel that are the result of sediment deposition as material gets transported downstream.
Main Channel Width: A measurement of a river channel at a certain point from bank to bank. It is measures perpendicular to the center line of the channel.
Side Channel Length: The length of an inundated (filled with water) channel that branches off the main channel of a river.

All of these features when looked at together are referred to as channel complexity, and can be used to indicate the health of a river system and the wildlife it supports.

Aerial imagery of the Dolores River, with arrows pointing to deposition on bank-attached bar, erosion of cut bank, channel width (boxed in red), mid-channel bar, and side channels (also boxed in red).

Unvegetated Gravel and Sand Bars

The number of unvegetated gravel and sand bars has decreased. This trend is worrisome because many fish species prefer 'clean' cobble bars for spawning. This loss of bar area happens for two different reasons:

Just below the dam, released water is devoid of sediment, so existing sandbars are stripped away by high flows and not replenished by sediment deposition. In other words, there is not enough sediment to maintain sandy bars.
As you move downstream (especially downstream of Disappointment Creek), we start to see the opposite problem - too much sediment, not enough water. Tributary flows driven by summer storms or low-elevation snowmelt add a lot of fine-grained sediment (sand, silt, and clay) to the river, where it is deposited onto bars and channel margins. If there are no high flows on the Dolores to move that sediment downstream, it plugs up the bars the fish might have used to spawn. This process is enhanced by vegetation encroachment onto the bars during low-flow years. The vegetation helps to trap and rapidly colonize the sediment, and eventually the bars become part of the vegetated floodplain rather than the active channel. This leads to an overall decrease of the channel width through time.

Main Channel Width

Main channel width is decreasing. This trend is more pronounced as the Dolores meanders northwest downstream from the McPhee Dam. Lack of high flow events that flush sediment out of bars/banks and vegetation encroachment decrease the mobility of a main channel within a floodplain, locking its path in place. This causes increased downcutting (incising) into the material that holds the main river channel. This in turn decreases the channel's width. The following infographic demonstrates this relationship using data collected from aerial imagery.

The largest loss in channel complexity can be found in the Below Disappointment Creek study segment, which shows a marked loss in main channel width (55%) and side channel length (66%).
This is contrasted by the Below San Miguel study segment, which seems to have retained the most channel complexity out of all the study segments, losing only 31% of its previous channel width and 28% of its side channel length.
The most nuanced changes in river complexity are seen in the Bradfield study segment, which has shown an increase in main channel width (20%) but also the most loss of side channel length (72%). This pattern supports a hypothesis of a widening channel that completely inundates side channels and washes away the sandbars that define them, carrying their sediment away to be deposited downstream in areas that have lost main channel width.

Side Channel Length

The number of side-channels on the Dolores River is decreasing. This metric captures a trend where in side-channels of the river become abandoned and filled with sediment and vegetation. This loss of channel complexity does not bode well for young fish, who use side channels as a way to hide from predators.

Side Channel Length in Each Study Segment

One way to measure channel complexity is by measuring the total length of inundated (flooded) side channels in a segment of river. In 2021, FLC students Charlie Brockway and Jack Tingwall traced all side channels in the 1982 (pre-dam) air photos and in satellite images collected during a controlled release in 2019. This study provides some answers to the questions posed above.

The plots below show side channel length in six segments of the Dolores River. Blue dots and lines show how side channel length changes with increasing discharge during the 2019 release. The orange dots show side channel length at ~1600 cfs in 1982 for comparison.

Each subplot shows a segment of the river, showing the side channel length at different discharges.
The blue dots and lines show how side channel length changes during a controlled release out of McPhee Dam.
In some tighter canyon segments (like Ponderosa Gorge and Slickrock Canyon), side channel length doesn’t seem to change with discharge at all.
In other, wider valley segments, like the Below Disappointment Creek and the Below San Miguel segment, side channel length increased as the discharge increased.
The greatest increases were seen as discharge rose above 2000 cfs, especially below the tributaries of Disappointment Creek and Below San Miguel. This might be a good target zone for future releases as a lot of habitat seems to open up at that level.
The orange dots show the side channel length in the same segments of river from the 1982 aerial imagery (which was collected at ~1600 cfs).

Graphs showing the side channel lengths according to discharge in c f s, between 1982 and 2019. Details above.

Research in Progress

Infrared imagery of the Dolores River at the slick rock study site.

This animation shows the Slickrock study site located downstream of McPhee Dam. Remote sensing techniques are used to show how the colors display or "pop-out" in the imagery to help distinguish between water and vegetation. The dark gray/blue segment is the river channel, and the red portions are vegetation.The imagery shows how the channel length changes at different flow rates from floods in June 2019. Notice how higher flow rates correspond with more side channels.