The Arroyo Problem Revisited: Valley Fill Stratigraphy and Adventures in Southwestern Watershed Rest
The following research was published in "The Outcrop," a monthly magazine sponsored by the Rocky Mountain Association of Geologists in October of 2022. The study is part of Seth Mangini's research as a PhD candidate at Montana State University.
THINK OF THE SOUTHWESTERN US AND WHAT COMES TO MIND?
Colorful, rocky, desert landscapes? High mountain peaks covered in snow? Vast grasslands? Deep canyons? Ancient pueblos? Roadrunners? Coyotes? Think of any of these and you will be correct. The southwestern US is a diverse landscape geologically, ecologically, and culturally. One landscape feature that many people will notice is the ubiquitous presence of incised stream channels. In Spanish the word arroyo means brook or stream. In the southwestern US the term refers to a channel, often ephemeral, which has incised into fine-grained cohesive valley fill to form a gully which has vertical walls and a flat bottom (Fig. 1). Some arroyos can be upwards at 30m (100’) deep. Arroyos form when a stream initiates a knickpoint or “headcut” which propagates upstream through a valley bottom (Fig. 2). These channel forms have been the subject of a great deal of academic study, including my own current PhD work. The processes and effects of channel incision intersect with the ecology and management of riparian systems in the arid southwestern US where long-term drought and a warming climate make the retention of water on the landscape and resilience of riparian systems critical issues. This article will discuss my PhD work on Bonito Creek in northeastern New Mexico. Bonito Creek is a headwaters stream and wetland system on the eastern slope of the Sangre de Cristo Mountains at ~9000’ in elevation (Fig. 3). The wetland system is comprised of wet meadows and peat forming fens (Fig. 4). The Bonito Creek wetlands exist at the southern end of the range for these types of systems, which are more common in places like Colorado, Montana, and Canada. As such, Bonito Creek will be a bellwether for the effects climate change will have on these wetlands.
“THE ARROYO PROBLEM” AND THE ARROYO CYCLE
Arroyo cutting can occur rapidly and dramatically change the morphology of a valley bottom. An infamous summer monsoon in Silver City, NM in 1887 initiated downcutting which turned its main street into a chasm 30 feet deep (Fig. 5). Studies have shown that many channels in the southwest have undergone repeated episodes of incision and refilling during the Holocene. Most of the arroyos on the landscape today were cut during the period of ~1870 – 1920 and the rapidity by which arroyo cutting occurred was noted by many writers at the time. This spurred a great deal of academic interest in the causes of “the arroyo problem” and many geomorphologists, including Stanley Schumm and Luna Leopold worked on the topic during the following decades. Academic debate on the topic has mainly been around whether the modern episode of channel cutting was due to land use change by Euro-American settlers and whether pre-historic episodes were caused by autogenic sedimentary processes or are related to external climate forcing. This is a debate which has never been fully resolved.
WOODY DEBRIS AND BEAVERS: AN EVOLUTION IN THE UNDERSTANDING OF RIPARIAN SYSTEMS
The field of riparian restoration has evolved rapidly in the past 10-15 years. Traditional river restoration techniques focused on shaping channel forms to a desired end state, often with the use of heavy machinery. This is slow, expensive, and overlooks the fact that healthy rivers are dynamic systems with channels that constantly move. It is estimated that >75% of river miles in the continental United States have been impacted by human activities and one third are listed as impaired by US EPA. The New Mexico Environment Department estimates that nearly half of the state’s wetlands have been destroyed since the beginning of Euro-American settlement. To have meaningful impacts, restoration needs to be large scale and low cost. Recent research has documented that accumulated woody debris and damming by beavers played an outsized role in shaping the morphology of waterways historically. The removal of beavers (by trapping) and woody debris (for instance by overgrazing of willows and cottonwoods) has created riparian systems which are less complex and more efficient at transporting water and sediment. This has resulted in rivers which are more prone to erosion, have less water storage capacity, and have less ecological diversity. As a result of this new understanding the restoration community has begun to shift towards Process-Based Restoration (PBR) methods which emphasize removing causes of degradation and reintroducing sources of morphological complexity to riparian systems. Introducing complexity is intended to set in motion processes which allow the river to evolve to a restored state over time under its own power. One PBR technique which has been gaining in popularity is the Beaver Dam Analog (BDA). The BDA is essentially a human constructed beaver dam. Large numbers of BDA’s can be installed quickly and inexpensively. They create surface and groundwater storage and can help capture sediment to aggrade incised channels. They are often colonized by actual beavers which then maintain and expand them. The role which beaver historically played in shaping the morphology and ecology of North America’s rivers and streams has only recently been appreciated. An estimated 400 million beavers once inhabited North America. The 18th and 19th century fur trade nearly caused their extinction with profound consequences for North America’s streams. By 1900, it is estimated that only around 100,000 still lived on the continent. Today populations have rebounded to around 10-15 million, which is enough to prevent their extinction, but not enough to fully fill their previous role in modifying our riparian landscapes. Process-based restoration aims to use beaver and woody debris to reverse this. Among the benefits of beaver colonization of streams include higher base flows, lower water temperatures, and the creation of natural riparian fire breaks.
Bonito Creek is located on Philmont Scout Ranch, a 140,171-acre property owned and operated by the Boy Scouts of America. Each year approximately 20,000 teenage participants from around the country come for 7 – 21-day backpacking trips. The ranch was donated to the BSA by Waite Phillips, an oil tycoon and philanthropist in 1938 as a place for young people to challenge themselves in the outdoors. The ranch is operated by a staff of 75 full time employees and 1400 seasonal staff. This includes a Conservation Department of 120 staff members who maintain trails and conduct natural resource management. It was as a seasonal conservation staff member that I first became involved with Bonito Creek In 1982 the Philmont Conservation Department hired a crew to build erosion control structures on arroyos in Bonito Creek. They built one large prototype structure, but the project was not continued. Further interest in restoration would not happen until 2010 when, Mark Anderson, Philmont’s Program Director, tasked myself and another staff member with conducting a survey of Bonito Creek. My background was in trail design and forestry, and I had little experience in stream morphology, so Mark bought us some books and sent us to some restoration workshops. We spent two months measuring the creek’s longitudinal profile and documenting headcuts and other erosional features. We concluded that the creek had significant erosion issues and would benefit from further monitoring and restoration efforts. Mark also suggested that perhaps the project could be led by a graduate student, an idea which was to stick with me. I left the ranch in 2012 to pursue my master’s at Montana State University and then work for the US Forest Service, but the idea of a restoration project on Bonito Creek was always in the back of my mind. In 2017, I approached Dr. Jean Dixon, a geomorphologist at Montana State, about becoming my PhD advisor researching arroyo processes and restoration on Bonito Creek. I reentered graduate school in the spring of 2019 and the project was fully funded by EPA 319 Clean Water Act grant beginning in 2021.
THE BONITO CREEK PROJECT
There are three components to my PhD: (1) a study of morphology and stratigraphy of Bonito Creek to determine its pre-disturbance conditions, the age of its wetlands, and what pre-historic processes influenced the evolution of the watershed; (2) a study comparing the timing of arroyo cutting in Bonito Creek to other nearby entrenched watersheds; and (3) the design and implementation of a process-based restoration program for Bonito Creek designed to reverse the entrenchment of arroyos, restore wetlands, and build resiliency to drought and climate change. Research questions which I hope to answer in my first study are how long it took wetland sediments to accumulate, whether the valley had undergone previous episodes of incision, and what geomorphic factors account for the formation of these unique wetlands. We are using a novel combination of study methods including historic aerial photographs (to determine the rate of arroyo growth in historic times) (Fig. 6), physical geomorphic measurements to look at system morphology (i.e. - longitudinal profiles, cross sections, entrenchment rations, etc.), pits and core samples to study sediments and stratigraphy (Fig. 7), fallout radionuclides and radiocarbon dating to measure the age of stratigraphic layers, ground penetrating radar (GPR) and electronic resistance tomography (ERT) to look at valley stratigraphy and water table depth. The second component of the study involves comparing the stratigraphy of Bonito Creek to other arroyo forming systems in the area. The goal of this part of the project is to help shed light on whether arroyo entrenchment is related to autogenic sedimentary processes or allogenic (i.e. – climatic) drivers. We are specifically looking at another entrenched system, North Ponil Creek, which lies about 10 miles to the north of Bonito Creek (Fig. 8). This part of the study also involves a review of published stratigraphic studies of other watersheds in the region which specifically look at the timing of entrenchment though the Holocene. The third part of my dissertation is the development and implementation of a restoration plan and monitoring regime for Bonito Creek. The development of an actionable restoration plan was critical to obtaining EPA funding for the project. I believe that a key factor driving erosion in Bonito Creek has been the removal of woody riparian vegetation such as willows from the system. Today the valley floor hosts almost no willows, except for one area fenced to exclude livestock. Digging in valley floor, however, one finds a great deal of woody debris (Fig. 9). This is supported by other studies in similar systems. This removal was likely facilitated by grazing and browsing by both domestic livestock and elk.
Actual restoration work is wet, muddy, and of course fun. The restoration plan has three parts: (1) halting of erosion at headcuts; (2) the restoration of channel complexity using process-based restoration methods; and (3) fencing parts of the valley to prevent grazing and browsing by domestic livestock and elk. The halting of erosion is being accomplished by the construction of stone structures (termed “Zuni bowls” because they were developed in restoration work on the Zuni reservation in western NM) (Fig. 10). These structures turn headcuts into hardened stone waterfalls and prevent their upstream migration. The second part is using BDA’s and other process-based restoration treatments to begin the process of restoring channel complexity, refilling incised reaches of the valley, and recharging groundwater (Fig. 11). Seven-foot fencing will keep elk and domestic livestock out of the stream bottom and allow woody vegetation to regrow (Fig. 12). Restoration work began in 2021 and is being done by a five-person crew hired with EPA funding as well groups of scouts and other volunteers. Work will include 15 Zuni bowls, ~150 BDAs, and ~5 miles of exclosure fencing. The first phase of restoration work will be completed in June of 2024. Monitoring will be critical to determine the effectiveness of restoration treatments. Monitoring involves: (1) repeated drone flights and photogrammetry of the valley floor to measure how treatments change valley topography and vegetation (Fig. 13); (2) repeated measurements of cross sections and the longitudinal profile; (3) a stream gauge and precipitation gauge to measure stream discharge; (4) a water quality monitoring station and sampling regime to measure nutrient loads, dissolved oxygen, turbidity, E. coli, and general water chemistry; and (5) monitoring wells paired with repeated ERT measurements to measure changes in groundwater retention related to restoration treatments.
Restoration work on Bonito Creek will take decades, but hopefully this project will provide important background on the geological processes which drive watershed evolution as well as practical knowledge on the implementation and effects of restoration work. It will also hopefully inspire future geologists and restorationists to grab shovels, dig holes, and ask questions.