Figure 1. Arroyo morphology as seen in Chaco Arroyo, New Mexico. The arroyo comprises an incised channel and erosional and depositional geomorphic forms. From Love (1983).
The most striking process in arroyos is their alternation between periods of incision and aggradation. Following cutting, which is the present state of Chaco Arroyo seen in fig. 1, arroyos slowly fill or aggrade with sediment. This "arroyo cycle" as portrayed by Gellis (1992) in fig. 3 is a common phenomenon throughout the southwest (see "The Arroyo Problem in the Southwestern United States"). The relation of the arroyo cycle to climate change is not clear at this time. Despite our uncertainty about the future direction or rate of climate change, history does provide lessons about the magnitudes of landscape change within arroyos. This paper presents a short introduction to the history of the Rio Puerco, New Mexico, and the effects of it's instability on past populations. Future effects are under study through a combination of sediment monitoring and modeling of arroyo processes.
Figure 2. Location of the Rio Puerco drainage basin, New Mexico
Figure 3. Arroyo evolution: stages A through G can represent changes at a cross-section through time or changes from the lower reaches to the upper reaches of the watershed at any point in time. From Gellis (1992).
Despite the potential for rapid changes in arroyos, the arroyo cycle and its relation to climate is still not adequately understood for the most recent geologic period, the Holocene. In the last 10,000 years, there have been repeated cycles of arroyo cutting and filling (Cooke and Reeves, 1976; Gellis and Elliott, in press; Dean,1994). Figure 3 (from Gellis, 1992) shows the stages in the arroyo cycle for a typical southwestern drainage basin. In the southwest, Gellis and Elliott (in press) have documented that cutting was most prevalent between 400-800 years before present (ybp), 1900-2700 ybp, 4400-5400, and 6500-7400 ybp. The non-random distribution of cutting episodes with time over large areas suggests that some regional forcing mechanism is responsible for the crude synchroneity. However, if climate change is the forcing mechanism, the state of the arroyo system before a climate change ensues may also be critical.
Dean (1994) argued that the aggradation/degradation cycles on the Colorado Plateau have averaged about 550 yrs. Pavich (unpublished data) has evidence from 14C dating of alluvium that 500-year and longer gaps exist in the record of alluvial fills in the Rio Puerco. These gaps also correlate with gaps in the ages of pack-rat middens in New Mexico (Betancourt et al., 1993). Although the data-base is small, these gaps in arroyo sediment ages, interpreted to be periods of enhanced cutting or incision, show that the Rio Puerco 14C-dated alluvial stratigraphy may correlate with other climatically related records.
|In a period of this
number of years:
|We can observe these changes:|
|10||Increases in thickness of aggraded alluvium, piping, gullying, bank collapse, density of riparian vegetation, all resulting in changes in channel geometry, El Nino cycles|
|100||Rapid arroyo incision and head cutting, development of incised, entrenched arroyo|
|1000||Arroyo aggradation, climate variations such as the Little Ice Age, Medieval Warm Period, major droughts in the west and southwest|
|10,000||Major climate changes such as the transition from a more humid to the semi-arid climate of the Holocene over the entire southwest.Beginning of semi- arid climate in the southwest|
|100,000||Major glacially-driven wet/dry cycle, cycle of rock weathering and sediment production|
Many people believe that the Rio Puerco underwent a transition from a well vegetated, unincised stream to a barren, arid wasteland since the latter part of last century. This transition is usually attributed to incision of the main channel below the valley floor and subsequent lowering of the water table and loss of vegetation. Incision is attributed to either overgrazing or a climatic shift (Bryan, 1928; Bailey, 1935). Nearly a century of scientific debate has focused on the precise cause of that incision. Incision and subsequent changes in the landscape have also led to legal action, including a recent act of congress calling for a restoration of the Rio Puerco to its "original state". However, it is not clear what the Puerco's "original state" may have been and it is equally unclear what natural changes would have taken place in the river without the influence of man. In order to provide information relevant to these issues, an attempt is underway to compile historical data on vegetation, land use, and channel conditions within the Rio Puerco basin during the recorded history of the region. What follows is an outline of information gathered on the state of the Puerco's main channel so far.
The picture that early Spanish explorers paint of the Rio Puerco in the 17th and early 18th centuries is one of a relatively well-vegetated area. In the few descriptions of this river that have survived, the Cottonwoods along its banks are invariably mentioned, and they were numerous enough that a legal dispute arose as to who owned the rights to their timber (Lopez, 1980). Today cottonwood trees are entirely absent from most reaches of the river. De Vargas, in 1692, named the Rio Puerco "La Torriente de los Alamos". This name again suggests that Cottonwood trees ("Alamos") were a prominent feature along the river's banks and also indicates that the Rio Puerco has flowed fast and muddy (Como un Torriente) for centuries. Indeed, the Puerco was dry when De Vargas first crossed it, but upon returning his men had to hurry to ferry supplies as the river was quickly rising, presumably in response to local rainfall. Such "flashy" flows are still common in the arid Southwest and often promote stream incision. Residents of Los Quelites (settled in 1765 near the confluence of the Rio Puerco and Rio San Jose) discovered this erosive power when a gully developed in their fields shortly after settlement. The formation of a gully in the fields of Los Quelites indicates that the main channel was incised below the surrounding valley floor at this time. Unfortunaltely, no Spanish reports have been found that provide quantitative descriptions of the shape, width, or depth of the channel of the Puerco.
The Rio Puerco has had reaches that flowed within deep, vertical-walled channels (arroyos by definition) at least 30 feet deep since at least 1846. During this year Lieutenant James W. Abert twice crossed the river on a military reconnaissance. He found the channel to be 10-12 feet deep at a location west of Albuquerque and 30 feet deep upstream near the abandoned settlement of "Poblazon" (Bryan, 1928). In 1849 Lieutenant J.H. Simpson described banks up to 30 feet high between La Ventana and Cabezon. However, residents of the area near Cabezon remember the channel in some places between Guadalupe and Cuba as "insignificant" and small bridges (<8 feet long) were used to cross the Rio Puerco and Arroyo Chico (a major tributary that enters the Puerco near Cabezon) as late as approximately 1890. Irrigation was practiced in several locations along the entire length of the Puerco in the second half of the nineteenth century, and residents recall that one man could divert the stream into the community irrigation ditch by dropping a single cottonwood tree across the river. The existence of this type of irrigation system alone is conclusive evidence that the main channel was not everywhere deeply incised during the 1870's and early 1880's. A photograph of the Rio Puerco in flood by R.H. Chapman in 1905 shows the main channel to be unincised and flood flow is spread over at least a hundred meters of broad valley floor. It is unclear exactly where this photo was taken, but such a situation is impossible at all locations on the Rio Puerco today as flood discharge would not overtop the banks of the present arroyo. Portions of the Rio Puerco were "not deeply incised" even as late as 1935 and major tributaries were unincised at this time (Gorbach et al, 1996). Photos taken in 1916 and 1983 (Figures 4a and 4b) show the contrast of unincised and incised channels south of the AT & SF bridge (Hawley, Love, and Wells, 1983). Figures 5a and 5b contrast the incised channels near Cabezon in 1885 and 1977 (Schumm, et. al., 1984).
Figure 4a. The Rio Puerco south of the AT & SF railroad bridge; the highway bridge in 1916 (from New Mexico State Engineer's Office).
Figure 4b. The Rio Puerco south of the AT & SF railroad bridge; the same reach illustrated in Figure 4a, only taken in July, 1983 (photography from Doug Heath).
Figure 5a. The Rio Puerco near Cabezon in 1885 (photography by E.A. Bass).
Figure 5b. The Rio Puerco near Cabezon; the same reach illustrated in Figure 5a, only taken in 1977 (photography by H.E. Malde).
The only way to reconcile these conflicting accounts is to conclude that the Rio Puerco was not completely incised until some time in this century when a continuous arroyo system obtained. The successive abandonment of agricultural land and villages along the Puerco during the late 1880's and 1890's (Bryan, 1928) suggests that the bulk of main-stem arroyo integration/incision took place during this time.
As early as 1902 scientists were beginning to notice arroyos as prominent features of the Southwestern landscape and to speculate as to what caused their formation (Tuan, 1962). By 1927 the Rio Puerco was recognized as the largest sediment- producing tributary of the Rio Grande and was seen as a serious threat to the long-term usefullness of Elephant Butte Reservoir (Bryan and Post, 1927). If it is accepted that the Rio Puerco entrenched its valley floor during the late 1880's through the early 1900's then one would expect a large increase in sediment production during the first decades of this century as thousands of tributaries to the Puerco incised their channels in an effort to "catch up" with the main stem. Erosion within the Puerco valley has been a constant concern of land-management agencies ever since. Thousands of earthen erosion control structures have been built in tributary basins of the Puerco and tamarisk (or Salt Cedar) were introduced to the valley in 1926 for their bank- stabilizing ability, but no large-scale effort has ever been undertaken to "rehabilitate" the main channel. Despite the lack of direct remediation measures on the main stem, sediment loads and peak discharge near the mouth of the Puerco have consistently declined since the late 1940's (Gellis, 1991). It is not possible to say with certainty what has caused this decline. Reduction in the amount of grazing, bank stabilization by Tamarisk and subsequent sediment storage, successful erosion control structures, and natural evolution of the arroyo system may all have contributed. Despite this decline in total sediment production, the Rio Puerco still supplies approximately 78% of the sediment entering Elephant Butte Reservoir. Headward erosion of tributary arroyos is very active and huge amounts of sediment are derived from caving of arroyo walls both along the main channel and on tributaries.
The continuing effort to compile historic information on the Rio Puerco will help to answer questions of interest to scientists, ranchers, land-managers, and residents of the Puerco Basin. In the long term, it may be possible to separate the effects of climate change from those of human activities on the evolution of the Rio Puerco and other arroyos in the Southwest.
Through field studies and modeling we are addressing questions such as:
- What is the annual and peak discharge response to El Nino extremes?
- What is the sediment load response to El Nino cycles?
- If the arroyo is unstable under a "constant" climate, how will it respond to a more variable climate?
In addition to climate-related questions, there are practical questions about the relation of arroyo processes to land-use. Two useful links for more information in the Rio Puerco region are:
- The importance of arroyos to urban land-use can be found in
Albuquerque's Environmental Story.
- The relation of land cover and hydrologic processes to El Nino cycles is under study at the Sevilleta LTER.
Bailey, R.W., 1935, Epicycles of erosion in the valleys of the Colorado Plateau Province: Journal of Geology, v. 63, p. 337-355.
Betancourt, J L., Pierson, E.A., Rylander, K.A., Fairchild-Parks, J.A. and Dean, J.S., 1993, Influence of history and climate on New Mexico Pinon-Juniper woodlands, in: Managing Pinon-Juniper Ecosystems for Sustainability and Social Needs, E.F. Aldon and D.W. Shaw, eds., USDA Forest Service General Technical Report RM-236, p. 42-62.
Broecker, W.S., 1997, Will our ride into the greenhouse future be a smooth one?: GSA Today, v. 7, no. 5, p. 1-7.
Bryan, K., 1928, Historic evidence on changes in the channel of the Rio Puerco, a tributary of the Rio Grande in New Mexico: Journal of Geology, v. 36, p. 265-282.
Bryan, K, and Post, G.M., Erosion and control of silt on the Rio Puerco, New Mexico: (unpublished) Report to the Chief Engineer State of New Mexico Middle Rio Grande Conservancy District, Albuquerque, New Mexico, 173 p.
Bull, W.B., 1991, Geomorphic Responses to Climate Change, Oxford University Press, New York.
Cooke, R.U. and Reeves, R.W., 1976, Arroyos and Environmental Change in the American South-West, Clarendon Press, Oxford, 213 p.
Dean, J. S., 1994, The Medieval Warm Period on the Southern Colorado Plateau, in: Hughes and Diaz, eds., The Medieval Warm Period, Climatic Change, v.26, p. 225-242.
Gellis, A.C., 1992, Decreasing trends of suspended sediment concentrations of selected streamflow stations in New Mexico, in: Proceedings of the 36th annual New Mexico water conference- agencies and science working for the future, p.77-93, New Mexico Water Resources Research Institute Report 265.
Gellis, A.C. and Elliott, J.G., in press, Arroyo Changes in Selected Watersheds of the Southwestern United States, in: Gorbach,C., Love, D., Piper,S., Davis, R., and Cross, A., (unpublished) Rio Puerco sedimentatin and water quality study: Preliminary findings report: U.S. Department of the Interior Bureau of Reclamation, Albuquerque Area Office, 40 p.
Hawley, J.W., Love, D.W., and Wells, S.G., 1983, Summary of the Hydrology, Sedimentology and Stratigraphy of the Rio Puerco Valley, in: Chaco Canyon Country: a field guide to the geomorphology, Quaternary geology, paleoecology and environmental geology of northwestern New Mexico, S.G. Wells, D.W. Love and T.W. Gardner, eds.
Lopez, L., 1980, The founding of San Francisco on the Rio Puerco: A Document: New Mexico Historical Review, v. 55, p. 71-78.
Love, D.W., 1983, Quaternary facies in Chaco Canyon and their implications for geomorphic-sedimentologic models, in: Chaco Canyon Country: a field guide to the geomorphology, Quaternary geology, paleoecology and environmental geology of northwestern New Mexico, S.G. Wells, D.W. Love and T.W. Gardner, eds., p. 195-206.
Love, D.W. and Young, J.D., 1983, Progress report in the late Cenozoic geologic evolution of the lower Rio Puerco, New Mexico Geological Society Guidebook, 34th Field Conference, p. 277-284.
Schumm, S.A., Harvey, M.D. and Watson, C.C., 1984, Incised Channels: morphology, dynamics and control, Water Resources Publications, Littleton, CO, 200 p.
Tuan, Y., 1966, New Mexican gullies: A critical review and some recent observations: Annals of the Association of American Geographers, v. 56, p. 573-597.