Watershed fragmentation in Coastal Plain Rivers

Abstract

This study explores the origin of 15 small coastal watersheds (SCWs) confined entirely to the lower Coastal Plain, which lie between the watersheds of the major rivers flowing across the Texas Coastal Plain. The relationship between SCWs and larger rivers was examined to determine whether the SCWs developed independently of the larger rivers, or became separated from them due to drowning of confluences by sea level rise or watershed fragmentation avulsions. None of the SCWs show evidence of developing independently of larger drainages. In 11 cases, the SCW main streams were connected to each other and/or the larger rivers at lower sea level stands. Ten of these former confluences occur under current estuarine systems, indicating that these rivers were connected within the Holocene. In one case, the former junction is on the current continental shelf, where mapping of the shelf shows that at sea level lowstands associated with the last glacial maximum, the current eight large river basins and 15 SCWs were aggregated into four large watersheds. Watershed fragmentation avulsions in the lower Rio Grande, Colorado, and Brazos (2) Rivers are responsible for the other four SCWs. Avulsions are generally common in the study area, but for a channel shift to result in watershed fragmentation three conditions must occur. First, a successful avulsion (i.e., the new channel becomes dominant) must occur, involving the entire reach from the avulsion site to the river mouth. Second, hydraulic connection between the newer and older channels must be lost. Third, the abandoned channel must receive sufficient tributary input or upland run-off to maintain its path to the coast and avoid infilling.