THE INTERACTION BETWEEN VEGETATION AND CHANNEL MIGRATION ALONG A LARGE ALLUVIAL RIVER: BRAZOS RIVER, TEXAS, USA

Changing land use over time has resulted in the land cover nearly completely altered from the original vegetated patterns along many rivers in Texas. Recent attempts by the State of Texas to regulate the flows of many rivers of Texas are causing considerable problems with patterns of deposition and erosion in and along the channel of many of these rivers. It has become obvious that to ensure proper planning and management of these rivers requires understanding the relationship between changing flows combined with rates of landcover change. More specifically, a detailed understanding of discharge-driven rates of migration is fundamental.

The middle Brazos River traverses from the Blackland Prairie to the Interior Coastal Plain physiographic provinces in central Texas. The floodplain of the Brazos is composed of predominately silty sand with occasional outcrops of limestone. Today the activity of this length of the river is very different than what is was in the past. Thus, we assessed which is the greater control on channel stability: changes and magnitude in discharge or removal and alteration of the riparian vegetation?

Rates and magnitude of lateral channel migration occurring along a section of the middle Brazos River from the late 1920s to 2008 were mapped using aerial photographs, planimetric maps, and fieldwork. Based on our methodology, we found that long stretches of the Brazos River are completely devoid of substantial vegetation, whereas other reaches contain a matrix of vegetation, not entirely indigenous to the local ecosystem, which extends only a few meters from the edge of the bank. We examined this change of rates of channel migration through several decades. We were not concerned how channels and vegetation interact under natural conditions, but rather what is the effect of private land management on lands along alluvial riparian rivers. Channel migration rates for the Brazos River average 4 (+/- 3) m/year. Reaches with the highest rates of migration correspond to riparian areas stripped of natural herbaceous vegetation.