Historic photographs and records show profound transformation of a dryland river over the last 100 years
Moenkopi Wash once was a wide, meandering river with large floods. Now it is less than half its former width, and nonnative vegetation crowds its banks. What mechanisms caused this transition, and what effects does this have downstream? A new study explains.
Moenkopi Wash winds ~95 miles through the Navajo and Hopi Reservations in northern Arizona on the Colorado Plateau, originating northeast of Tuba City and flowing south to Cameron, draining into the Little Colorado River. It once flowed along wide channels, moving large amounts of sediment through a river valley with small, undeveloped floodplains. It is the primary provider of sediment to the Little Colorado River, which in turn delivers much of that sediment into the Colorado River.
A new study examined historic photographs, aerial imagery, and discharge and sediment records over the last century to analyze how the channel has changed, the processes responsible for the change, and the resulting effects.
Historic discharge records between 1926 and 1940 show frequent flooding in Moenkopi Wash during the summer and fall monsoon season. This period was characterized by some of the largest floods ever measured in Moenkopi Wash. After 1940, a change in the regional climate, likely resulting in less rain, resulted in smaller floods. Although floods were smaller, the channel remained wide, but vegetation began to establish along the channel margins. The reduction in flooding alone did not change the size or meandering nature of the channel.
Early in the 1900s, tamarisk (Tamarix spp.), a nonnative, drought tolerant thicket-forming tree was introduced for erosion control into the Little Colorado Basin. Photographs taken in the 1940s do not show signs of tamarisk along Moenkopi Wash. Those taken in 1959 show low to moderate density of tamarisk, so it likely spread throughout the Wash during the 1950s.
After 1955, another likely decline in rainfall further reduced flooding and allowed tamarisk to gain a stronger foothold along the channel banks and floodplains. As vegetation density increased, more sediment became trapped within it, resulting in channel narrowing. Thus, the initial decrease in flooding after 1940 had little effect on the width of the channel, but the second decrease in flooding after 1955 resulted in channel narrowing because vegetation was able to help trap sediment and stabilize the banks. Analyses of aerial photographs taken in 1952 and 2019 show that the wash narrowed by 57-59%. As the channel narrowed, the wide, snake-like curves that used to convey large floods became narrow ribbon-like threads winding through densely vegetated floodplain forests. As the channel filled with sediment, the river cut meander bends in some places, resulting in narrow, straight sections of river.
Tamarisk encroachment along the margins of Moenkopi Wash has now substantially altered the rate at which floods can flow because vegetation slows flood movement downstream. Furthermore, tamarisk reduced the amount of transported sediment because it traps sediment on the banks. The mere presence or absence of vegetation can have profound effects on channel morphology and hydrology (channel shape, conveyance capacity of floods). Historic sediment concentration records show that there's been a large reduction of sediment delivered to the Little Colorado River. Combined, the biogeomorphic effects of reduced flooding and nonnative vegetation encroachment caused a positive feedback; as the channel narrowed, overbank flooding and sediment deposition on the banks occurred because of a loss in conveyance capacity of floodwater, and vegetation caused additional declines in flow velocity, resulting in additional overbank flooding and trapping of sediment leading to more channel narrowing (see before/after photos).
Through a detailed analysis, this study found that vegetation plays a stronger role than hydrology alone in causing geomorphic change. As a result, fine sediment delivery into the Little Colorado River and Colorado River downstream, where sediment is a key management resource, is a fraction of what it was historically. Without record floods that remove vegetation or large-scale mechanical vegetation removal, Moenkopi Wash is unlikely to return to its earlier, wide-channel meandering state.
Read the paper:
Dean, D.J., and Topping, D.J., 2024, The effects of vegetative feedbacks on flood shape, sediment transport, and geomorphic change in a dryland river—Moenkopi Wash, AZ: Geomorphology, v. 447, article 109017, p. 1-23, https://pubs.usgs.gov/publication/70250681
Associated data:
Dean, D.J., and Topping, D.J., 2023, Discharge, topographic, suspended-sediment, and GIS data from Moenkopi Wash, AZ: U.S. Geological Survey data release, https://doi.org/10.5066/P9TKO358
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