Characterization and evaluation of channel and hillslope erosion on the Zuni Indian Reservation, New Mexico, 1992-95

Like many areas of the southwestern United States, the Zuni Indian
Reservation, New Mexico, has high rates of erosion, ranging from 95 to
greater than 1,430 cubic meters per square kilometer per year. Erosion
on the Zuni Indian Reservation includes channel erosion (arroyo incision
and channel widening) and hillslope (sheetwash) erosion. The U.S. Geological
Survey conducted a 3-year (1992-95) study on channel erosion and hillslope
erosion in the portion of the Rio Nutria watershed that drains entirely within
the Zuni Indian Reservation. Results of the study can be used by the Zuni Tribe
to develop a plan for watershed rehabilitation.

Channel changes, gully growth, headcuts, and changes in dirt roads
over time were examined to characterize and evaluate channel erosion in the
Rio Nutria watershed. Channel cross-sectional changes included width, depth,
width-to-depth ratio, area, and geometry. Relative rates of gully growth,
headcuts, and changes in dirt roads over time were examined using aerial
photographs. Results of resurveys conducted between 1992 and 1994 of 85
channel cross sections indicated aggradation of 72 percent of cross sections
in three subbasins of the Rio Nutria watershed. Forty-eight percent of
resurveyed cross sections showed an increase in cross-sectional area and
erosion; nine of these are in tributaries. Some channels (43 percent)
aggraded and increased in cross-sectional area. This increase in cross-
sectional area is due mostly to widening. Channel widening is a more pervasive
form of erosion than channel scour on the Zuni Indian Reservation. The tops
of channels widened in 67 percent and the bottoms of channels widened in 44
percent of resurveyed cross sections. Narrow, deep triangular channels are
more erosive than rectangular cross sections.

Five land-cover types--three sites on mixed-grass pasture, two sites on
sites on unchained pi?on and juniper, one site on sagebrush, one site on
ponderosa pine, and two sites on chained pi?on and juniper--were each
instrumented with sediment traps between 1992 and 1994 to measure hillslope
erosion. Highest sediment yields were measured at chained areas and mixed-
grass pasture. Annual yields from sites that were operated for more than a
year were 11.7, 6.0, and 6.5 metric tons per square kilometer per year at a
pi?on and juniper site, mixed-grass pasture site, and sagebrush site,
respectively.