European settlement of the Big Bend and the accompanying alteration of the region’s watersheds have caused a series of cascading events that have resulted in significant degradation of the riparian environment. In a recent presentation at the Water in the Desert conference at Sul Ross State University, Price Rumbelow of Rio Grande Joint Venture explained that this degradation often “comes in the form of decreased structural complexity” resulting in “structural starvation” of riparian areas, which are the lands along the edges of creeks and rivers.
Factors, such as logging, have contributed massively to this structural starvation due to the degradation of our riparian forests. For example, the Study Butte Mining Company, which opened in 1902, purchased about 2,522 chords of firewood for its kilns over a three-year period, all of which was harvested from local sources. The U.S. Forest service defines a cord as “128 cubic feet, measured as a pile 8 feet long, 4 feet high and 4 feet wide. A full cord can weigh up to 5,000 pounds.” That is a lot of trees by any stretch of the imagination, especially if you consider our arid, desert environment. Fort Davis, with over 100 structures, was another heavy user of regional wood resources, resulting in severe depletion of riparian forests along the Cienega and Limpia Creeks in Jeff Davis County from 1867 to 1891.
The structural starvation of our riparian areas is also due to overgrazing. The Chihuahuan grasslands have supported herds of cattle, sheep and goats numbering in the tens of thousands from the 1500s to the late 1800s. According to Rumbelow, livestock “are typically limited by three things: food, water, and cover. When grazing occurs without guidance … livestock will follow the path of least resistance and seek to establish a home range with easy access to all three” in riparian areas. Over the decades, this has led to “fragmented riparian forests” with reduced populations of cottonwoods and willows, which can be “pretty obvious, if you know what you are looking for, [because] there are still plenty of old mature trees but almost no young or midstory trees,” explained Rumbelow. The result has been the accelerated exportation of sediment out of these riparian systems accompanied by “down-cutting” of the stream bed, where the incisive action of the water creates steeper slopes along the sides of the channel. The degradation cycle becomes self-perpetuating as sediment continues to be exported and the velocity of the water further down-cuts the stream bed over time. The rapid egress of sediment and water from the watershed actually decreases the volume of water that enters the underlying riparian aquifer, effectively reducing flows to the watershed and causing these once-perennial streams to stop flowing most of the year.
“One way of addressing this broken hydrology,” according to Rumbelow, “is with filter dams.” These simple structures, also known as “brush weirs,” comprise untreated posts that are pounded into pre-drilled holes in the stream bed. Local brush is then woven and stuffed between the posts to a height of 18 inches or so. Filter dams slow down the water and allow it to spread out, which decreases the scouring action of the rushing water and traps sediment. This trapped sediment not only builds up over time, “raising the floor of the channel,” it also carries seeds and nutrients that allow woody riparian species to take root. Perhaps most interestingly, Rumbelow explained that “the increased time that the water spends on the surface does really good things to the water below the surface. As the riparian aquifer fills, the base flows increase. The water output remains the same –– you can’t get more out of the system than went in –– but instead of the flows all occurring at once, they are spread out over time.” Riparian aquifers restored in this manner can then “be tapped by riparian vegetation, which in time will supply the woody debris to the systems that will eventually do the work of brush weirs, increasing structural complexity naturally.” Brush weirs are one of several tools that can remediate decades of damage and restore the health of our riparian forests, unleashing a host of benefits. “An intact riparian forest will aid in bank stability, sediment and nutrient retention, and increase structural complexity, ultimately increasing the natural storage of water in riparian aquifers,” explained Rumbelow.
Rio Grande Joint Venture recently secured a $3.5 million grant from the America the Beautiful Challenge administered by the National Fish & Wildlife Foundation. Some of these funds will be used to coordinate and incentivize stream restoration projects in the Big Bend region. Interested landowners can contact Price Rumbelow at 432.294.1168 or prumbelow@abcbirds.org for more information.
Trey Gerfers is a San Antonio native and serves as general manager of the Presidio County Underground Water Conservation District. He has lived in Marfa since 2013.