Uncommon twentieth-century stream behavior of lower Abo Arroyo revealed by flood deposits and historic photographs

Widely scattered coal and clinker clasts and flotsam, combined with historical photos, and modern observations in the lower Abo Arroyo indicate valley-wide flooding and significant changes in sediment transport and channel erosion during the 20th Century. Coal and clinker granules and pebbles (2to 40-mm diameter) as well as flotsam of whole juniper trees and other wooden debris along the valley floor of Abo Arroyo show that the valley was inundated by one or more large floods. Aerial photos taken in 1935 show widespread dark deposits along the valley floor. These dark deposits are believed to be coal that came from spills along the Atchison, Topeka and Santa Fe (AT&SF) Railroad through Abo Pass to the east. The railroad was built between 1903 and 1908, when the first timetable for the route was published. Prior to 1935, the most likely period for extensive flooding was in August and September of 1929, when regional storms and flooding occurred on the Rio Puerco, Rio Grande, and other gauged tributaries. The 1935 photographs also show that the lower 10 km of Abo valley had anastomosing unincised channels, gravel bars, and slackwater yazoos along the valley margins. No continuous incised channel existed, although branching short headcuts extended from the arroyo mouth at the Rio Grande, upstream about 2 km, across the valley-mouth Holocene alluvial fan. Aerial photographs taken in 1947 show that channel incision along the valley floor farther east had increased downstream and that much of the coal had been remobilized, reworked, and partially buried by later flood deposits. The lowest reach of Abo Arroyo became entrenched by 1954, completing a continuous channel from Abo Canyon westward. Incision by headcutting was, at most, a minor process at the arroyo mouth. This channel behavior along lower Abo valley is unlike most other arroyos in New Mexico because: 1) Abo Arroyo was not incised continuously well into the 20th Century, 2) due to concentration and then dispersion of stream power it incised downstream rather than upstream, 3) it is a bedload stream with a gradient three to seven times steeper than adjacent streams (Rio Grande and Rio Puerco), 4) adjacent sparse vegetation has minimal effect on flows, and 5) it is not evolving (so far) through stages of arroyo development and aggradation. 447