Hydrologic connectivity of oxbow lakes along the lower Guadalupe River, Texas: The influence of geomorphic and climatic controls on the “flood pulse concept”

Hydrologic connectivity between rivers and floodplains is fundamental to the ecological integrity of meandering river corridors. This study integrates two important concepts in floodplain hydrology; the flood pulse concept and the oxbow lake cycle. High temporal resolution (0.5–3.0 h) lake level data from vented pressure transducers and river discharge data (0.25 h resolution) were utilized to develop standardized indices of hydrologic connectivity for two oxbow lakes along the lower Guadalupe River, Texas over a 3.4 yr period. The lakes were influenced by hydrologic conditions representative of the northwestern Gulf region, notably a severe drought and a wet season associated with La Niña and El Niño, respectively. Because the data were obtained over the same period it enables a direct comparison of surficial hydrologic connectivity within the evolutionary geomorphic framework of the oxbow lake cycle. A recently (13 yr old) formed oxbow lake was connected 5.9 times per yr and an older partially infilled oxbow lake was connected 3.8 times per yr during the study period. Each oxbow lake was connected by discharge pulses below bankfull discharge, and the signature of hydrologic connectivity varied significantly. The flow duration for connectivity was 18% and 3% for the recently formed and older oxbow lake, respectively. Bivariate relations between discharge and lake stage varied significantly for the two oxbow lakes. The recently formed oxbow lake exhibited highly linear relations between discharge and lake levels, and lake drainage closely mirrored flood wave recession. Older oxbow lake connectivity was complex, as discharge and lake level relations were nonlinear and exhibited pronounced negative hysteresis, which increased with sequential flood pulses. While the older oxbow lake was more sensitive to short term low flow conditions, both lakes were completely desiccated during an extended La Niña influenced drought. This indicates a substantial impact to alluvial aquifers, which were unable to supply groundwater to oxbow lakes during extended dry periods. The “flood pulse concept” applied with only river discharge data is overly simplistic and does not adequately characterize hydrologic connectivity for distinct types of oxbow lakes along meandering river floodplains. With limited lake level data, however, great potential exists for development of effective “integrated floodplain management” plans that more appropriately considers variable hydrologic connectivity inherent within the oxbow lake cycle.