EFFECTS OF DISCHARGE VARIATION ON MEANDERS MIGRATION: MIDDLE BRAZOS RIVER, TX, USA

Meander migration has been investigated from numerous perspectives over the years. These investigations have resulted in an easy approach of applying this research to various rivers irrespective of size and style. Although these models are generalizations of a typical alluvial river, they do not fit all big rivers. Recent attempts to regulate Texas river basins necessitate detailed understanding of discharge-driven migration rates for proper planning and management. One of the key elements needed for appropriate management is an understanding of the rate of migration of meanders on a river. 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. The middle Brazos River traverses from the Blackland Prairie to the Interior Coastal Plain physiographic provinces. The floodplain of the Brazos is composed of predominately silty sand with occasional outcrops of limestone. The study reach consists of 72 bends and 203 km of channel. Flow regulation, which has resulted in substantial decrease in channel migration since the late 1940s, has greatly altered the discharge and suspended sediment characteristics of the river. Discharge from the late 1920s to 1949 averaged 75 cms whereas discharge averages 61 cms since flow regulation began. Peak flows occurred more frequently prior to regulation, impacting the rate and style of migration. Channel migration rates prior to regulation averaged 13 (+/- 3) m/year whereas migration after regulation averages 4 (+/- 3) m/year. Channel migration rates prior to aerial photograph coverage were calculated using dendrochronology of trees along scroll bar sequences. These migration rates are similar to those rates prior to flow regulation.