EVOLUTION OF A HYPERPYCNAL LACUSTRINE DELTA, RED RIVER/ LAKE TEXOMA, TEXAS-OKLAHOMA

The Red River where it flows into Lake Texoma provides a natural laboratory for studying how deltas evolve. This delta prograded into Lake Texoma more than 15 km since 1940 when the lake was impounded. Growth of the delta and change in its morphology was observed using aerial photos and satellite images collected from 1955 to 2002. Because basin energy is negligible compared with river energy, river processes dominate delta formation. A correlation between morphology variation and river discharge variation was established for hyperpycnal Red River delta built into Lake Texoma. From 1940 to 1981 annual discharge averaged around 100 m3/sec, with values of 50 m3/sec between 1958 and 1981. Annual discharge since 1981 averaged between 150 and 250 m3/sec with the exception of 4 years when it was between 50 and 100 m3/sec. When river discharge was low (pre 1981), the delta was lobate. When river discharge was high (post 1981) the delta is elongate (finger type). Input of sediments and delta progradation is controlled by river discharge. At 5 m3/sec water discharge the sediment flux is 1.8*103 m3/day. At extreme water discharge (1.2*103 m3/sec) the sediment flux is 9*105 m3/day. This volume of sediments approximates 150 m daily delta progradation into a basin similar to Lake Texoma (3 km wide and 2 m depth). River discharge also controls water density. Because Permian and Pennsylvanian evaporites deposits underlie much of the watershed, at low discharge the river water is saline (4.9 g/l TDS), river water is denser than lake water. During high discharges salinity is diluted by rainwater but, because suspended sediment concentration increases (21.1 g/l for a river discharge of 1022.2 m3/sec), river water is still denser than lake water. As a consequence of salinity/ suspended sediments interplay, the Red River frequently has a hyperpycnal type flow. Internal architecture of delta deposits can be inferred from delta morphology. During low discharges lobate shape will produces tabular, basinward dipping bodies, during high discharge more elongate, channel type bodies form.