Climate Change, Sediment Transport Capacity, Arroyo Development, and Vegetation Change in Streams of the Southwestern United States

Duration: February 1999 - September 2002

Arroyos are steep-walled, oversize channels cut by water into easily eroded sediment. In the late 1800's arroyos developed along channels of the Rio Puerco in New Mexico and many other streams of the southwestern United States. Along the Rio Puerco, arroyo incision drained surface water that formerly supported marshlands, riparian (streamside) woodlands, and associated grazing lands. It destroyed bridges and irrigation works, deposited large amounts of undesirable sediment in downstream rivers and reservoirs, and led to the abandonment of six towns and many ranches.
Since the 1930's, many arroyos, including the Rio Puerco, have partially filled. New riparian plant communities have become established; however, they are often dominated by the undesirable exotic shrub, tamarisk, instead of native cottonwoods and willows. Sediment loads in the Rio Puerco watershed are still heavy, and sediment deposition continues to reduce the storage capacity of reservoirs. Loss of grazing lands is also still a concern. Congress has directed the Bureau of Land Management to develop strategies to decrease erosion in the Rio Puerco watershed.

Arroyo cutting and filling have been attributed to shifts in the intensity of precipitation, changes in livestock grazing intensity, tamarisk invasion, and a cycle intrinsic to the landscape. Downcutting in the late 1800's occurred during a period of unusually high intensity rain events. The filling phase that began in the 1930's accompanied tamarisk invasion and changes in both land management and climate; however, the relative contributions of these three factors are not well understood.

Our project is aimed at determining how climate and land use influence: (a) the arroyo cycle, (b) associated vegetation changes, and (c) sediment transport. Given that some climate models predict more frequent climate extremes in conjunction with global climate change, this knowledge is likely to become increasingly important in coming decades.

Information from this study will help land managers assess potential outcomes of management actions in the Rio Puerco watershed under current conditions as well as under predicted climate changes.

Approach:

We will conduct our research at three spatial scales:

At the reach (section of arroyo) scale, we will use stratigraphy and tree-ring counts on 200 buried tamarisk stems to reconstruct the timing and amount of sediment deposition in 6 trenches across the Rio Puerco arroyo. This information will be related to historical records of stream discharge.

At the watershed scale, we will measure the cover of all plant species in 60 transects along the Rio Puerco and its tributaries. Using statistical techniques, we will relate the relative abundance of native and exotic species to:
water availability
livestock grazing intensity
the local stage of the arroyo cycle

At the regional scale, we will use a Sediment Transport Capacity Index to investigate how shifts in the intensity of extreme precipitation events are related to sediment transport and the arroyo cycle in fifteen streams on the Colorado Plateau, including the Rio Puerco, the Rio Grande and one or more tributaries to the Colorado River.

Applications of Results:

The watershed-scale work will help determine the extent to which land management actions can alter the arroyo cut-and-fill process and associated changes in vegetation. Important management issues include tamarisk control and the timing and intensity of grazing.

The regional-scale work will provide predictive models that will relate anticipated changes in precipitation intensity to changes in sediment transport. An improved understanding of the relationship between climate and the arroyo cycle will help water managers predict changes in the rate of sediment deposition in Elephant Butte Reservoir and elsewhere. Our work will complement other research being conducted by the USGS in the Rio Puerco watershed.

Information from this study will help land managers assess potential outcomes of management actions in the Rio Puerco watershed under current conditions as well as under predicted climate changes.

Products:

Products of this research will include briefings and presentations regarding the management implications of the results, presentations at scientific meetings, and publications in the peer-reviewed scientific literature.

Collaborators:

Milhous, Robert; Auble, Gregor T.; Scott, Michael L.; Shafroth, Patrick B.; Vincent, Kirk

	Primary Contact: Jonathan M. Friedman, USGS Fort Collins Science Center (FORT) [formerly Midcontinent Ecological Science Center (MESC)]