RECENT GEOMORPHIC CHANGE IN THE RIO PUERCO ARROYO OF NORTHERN NEW MEXICO, U.S.A

Stream incision into valley alluvium during the late 19th- and early 20th-centuries formed deep, continuous arroyos in the southwestern United States. Since the initial incision of the Rio Puerco, the arroyo has widened and in some reaches may be aggrading. Bryan and Post's 1927 observations and 1935 aerial photographs indicate that most of the 200-km-long Rio Puerco arroyo had enlarged by lateral erosion. By the 1950s and 1970s, the stream channel in the lower Rio Puerco was becoming narrower, and an inner flood plain within the arroyo was becoming stabilized by vegetation; however, these changes were not apparent in the upper reaches. Cross sections surveyed at 25 sites in 1977 were resurveyed in the mid 1990s. The channel at 12 sites in the upper Rio Puerco was characterized by a large width-to-depth ratio, a small amount of silt- and clay-size material in the channel perimeter, unstable and poorly defined channel banks, low vegetation density on the inner flood plain, a laterally unstable channel, and vertical arroyo scarps. The channel at 13 sites in the lower Rio Puerco were characterized by a small width-to-depth ratio, a large amount of silt- and clay-size material in the channel perimeter, steep and resistant channel banks, dense riparian-zone vegetation on the inner flood plain, a relatively stable channel position, and less common erosion of arroyo scarps. Most upstream sites continued to be unstable in the 1990s with lateral erosion of the arroyo scarps progressing since the 1970s; some upstream sites have aggraded and others have degraded. Downstream sites were relatively stable, but all had decreases in bankfull width and in width-to-depth ratio; 10 had decreases in cross-sectional area due to sediment deposition on the bed and banks. The channel at seven downstream sites had an increase in mean bed elevation of at least 0.3 m, and eight had a bankfull elevation increase of at least 0.3 m. Aggradation in upper arroyo reaches may occur where the relatively large channel width-to-depth ratio results in insufficient depth and shear stress to transport sand-size material. Channel aggradation in both reaches and width and area decreases in the downstream reaches also may be a result of transmission loss from infiltration leading to streamflow attenuation and sediment deposition. Nearly 80% of runoff is lost to attenuation.