New Mexico Geological Society Annual Spring Meeting — Abstracts

Widely scattered coal and clinker clasts and flotsam, combined with historical photos, and modern observations in ungaged Abo Arroyo indicate valley-wide flooding and significant changes in sediment transport and erosion during the 20th century. Coal and clinker granules and pebbles (2-40 mm diameter) and flotsam of whole juniper trees and other wooden debris along the valley floor of Abo Arroyo show that the valley was inundated by a large flood or floods sometime between 1911 and 1935. The coal must have come from spills along the ATSF railroad through Abo Pass to the east, a route that was completed in 1911 and had a number of wrecks of coal-powered steam trains transporting coal. Abo Arroyo drains westward from Mountainair, NM, to the Rio Grande. West of Abo Canyon its valley shows only a minor increase in contributing area for 25 km, and it is a hydrologically losing stream. Between 1911 and 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 gaged tributaries. Aerial photographs taken in 1935 show that the lower 20 km of Abo valley had anastomosing unincised channels, gravel bars, and slackwater yazoos along the valley margins that spread coal clasts across the entire valley. Aerial photographs taken in 1947 show 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. Once the arroyo was incised about 4 m, eolian processes on the valley floor created sheet sands and coppice dunes that buried most of the coal. Harvester ants between the dunes tend to collect granules of coal and clinker in their hills, while blow-outs and small mammal burrows expose larger clasts of coal and clinker.

The active depositional facies on the floor of the lower Abo valley after the coal floods and present exposures along the incised arroyo contrast with other streams in adjacent semiarid drainage basins. First, lower reaches remained unincised as recently as 1947 rather than being incised in the 19th century. Second, due to concentration of stream power, the arroyo extended incision downstream through time rather than by the common model of arroyo headwall cutting and migration upstream (except for the lowest 1-km reach). Of note, the gradient of Abo Arroyo is 0.0074, nearly three times steeper than that of the Rio Puerco (on the other side of the Albuquerque Basin) and seven times steeper than the Rio Grande. Third, although the alluvium of the valley floor is predominantly bedded sand, silt, and clay, the modern channel consists primarily of subangular to subrounded boulders and cobbles with temporary deposits of sand, silt, and clay on bars and pools left under waning flow. Fourth, grass is the predominant vegetation along this semiarid drainage--the modern channel and point bars have few large shrubs such as fourwing salt bush or tamarisk.