Abstract

In the summer of 2006, large floods occurred on the Red Canyon (footwall-derived) and Palomas Canyon (hanging-wall-derived) alluvial fans, which are located on opposite sides of the Rio Grande floodplain in the Palomas half graben of the southern Rio Grande rift. The initial phase of the Red Canyon flood took place on the active central fan lobe, but subsequent avulsion of the main channel shifted the flood to the south, where it eroded all but two small relicts of the former inactive, toe-cut lobe before depositing up to 2 m of coarse gravel. Deposition of 0.5 m of gravel-size detritus by the Palomas Canyon flood was largely confined to the active, incised channel and to gravel bars that built into the river, although floodwater spilled over the southern bank of the channel near its mouth, depositing a thin (< 15 cm) veneer of fine sand on the floodplain. Measurements of mean and maximum grain size of the flood deposits, channel-bed slopes, and flood depth based on vegetative high-water marks allow estimates of mean flood velocity (Red Canyon = 8.6 m s⁻¹; Palomas Canyon = 7.4 m s⁻¹), floodwater discharge (Red Canyon = 971 m³ s⁻¹; Palomas Canyon = 179 m³ s⁻¹), and volume of flood deposits (Red Canyon = 3.19 × 10⁵ m³; Palomas Canyon = 1.72 × 10⁴ m³).

Mapping of the modern floodplain and outcrop analysis of the Palomas Formation (Pliocene–lower Pleistocene) in the Palomas basin provide insight into facies architecture in half grabens. Footwall–derived alluvial-fan detritus consists principally of shallow, multistory gravel–conglomerate channels produced by channel avulsion across the surface of the small fans. Asymmetrical basin tilting causes toe-cutting of the footwall-derived fans by the axial river and incision of hanging-wall-derived alluvial-fan channels, the latter resulting in ribbon-shaped gravel–conglomerate channels interbedded with finer-grained, distal-fan sediment.

The Red Canyon and Palomas Canyon floods changed the location, width, and depth of the Rio Grande, causing flooding of the western floodplain for ~ 1 km upstream of the constriction. Because the Rio Grande has been dammed since 1916, it no longer has the stream power to effectively redistribute large volumes of fan detritus and must be dredged in order to avoid upstream flooding by the river.