[ad_1]
The Moenkopi River winds for about 95 miles through the Navajo and Hopi reservations on the Colorado Plateau in northern Arizona. It originates northeast of Tuba City and flows south to Cameron, where it empties into the Little Colorado River. It once flowed along a wide channel, carrying large amounts of sediment across the valley, which contained small, undeveloped floodplains. It is the primary source of sediment for the Little Colorado River, which in turn delivers much of the sediment to the Colorado River.
A new study examined historical photographs, aerial images, and discharge and sediment records over the past century to analyze how the waterway has changed, the processes that led to the changes, and the resulting impacts.
Historical flow records between 1926 and 1940 show that the Moenkopi River frequently flooded during the summer and fall monsoon seasons. During this period, the Moenkopi River experienced some of its largest floods ever recorded. After 1940, climate change in the region may have resulted in less rainfall, which led to smaller floods. Despite the smaller floods, the river channel remained wide, but vegetation began to grow along the channel edges. Reduced flooding alone did not change the size or meandering of the river channel.
In the early 20th century, Tamarix chinensis (Tamarix Tamarix (Tamaru spp.) is a non-native, drought-tolerant shrub introduced to the Little Colorado Basin for erosion control. Photographs taken in the 1940s show no traces of Tamarix along the Moenkopi River. Photographs taken in 1959 show low to moderate densities, so it was likely that Tamarix was widespread along the riverbank by the 1950s.
After 1955, rainfall dropped again, further reducing flooding and allowing the tamarisk to gain a stronger foothold on the riverbanks and floodplains. As vegetation density increased, more sediment was trapped within it, causing the channel to narrow. Thus, the first reduction in flooding after 1940 had little effect on channel width, but the second reduction in flooding after 1955 caused the channel to narrow as vegetation was able to help trap sediment and stabilize the riverbanks. Analysis of aerial photographs taken in 1952 and 2019 showed a 57-59% reduction in scour extent. As the channel narrowed, the wide serpentine curves that used to carry large floodwaters became narrow ribbons that meander through densely vegetated floodplain forests. As the channel filled with sediment, the river meandered in places, causing sections of the river to become narrow and straight.
Tamarix invasion at the edge of the Moenkopi River has significantly altered flood flow rates because the vegetation slows flood flow downstream. In addition, Tamarix reduces the amount of sediment transported because it traps sediment against the streambank. The presence or absence of vegetation can have a profound effect on channel morphology and hydrology (channel shape, flood-carrying capacity). Historical sediment concentration records show a substantial reduction in sediment inflow into the Little Colorado River. The combination of reduced flooding and the biogeomorphic effects of the invasion of non-native vegetation creates a positive feedback; as the channel narrows, overbank flooding and overbank sediment deposition occurs due to reduced flood-carrying capacity, while vegetation causes a further decrease in flow rate, leading to more overbank flooding and sediment trapping, which results in further channel narrowing (see before and after photos).
Through detailed analyses, the study found that vegetation played a larger role than hydrology alone in causing the changes in the landscape. As a result, the amount of fine sediment flowing into the Little Colorado and Lower Colorado Rivers—sediment is a key management resource—is only a fraction of what it was historically. Without record flooding that removes vegetation or large-scale mechanical vegetation removal, the Moenkopi River is unlikely to return to its earlier wide-channel meandering state.
Read the paper:
Dean, DJ and Topping, DJ. 2024. Influence of vegetation feedback on flood pattern, sediment transport, and geomorphic change in a dryland river—Mounkopiwash. Arizona: Geomorphology, Vol. 447, No. 109017, pp. 1-23. https://pubs.usgs.gov/publication/70250681
related data:
Dean, DJ and Topping, DJ. 2023. Discharge, topography, suspended sediment, and GIS data for the Moenkopi River, Arizona: U.S. Geological Survey data release. https://doi.org/10.5066/P9TKO358
[ad_2]
Source link
