Detrital Zircon Record of a Dammed River in Texas - Implications for Modern River Provenance Stories and Sediment Budgets

The Colorado River in Texas is a meandering river that is bisected by a chain of highland lakes and dams that were completed by 1951. Intuitively, dams trap sediment, but how does that disruption of sediment flow affect overall sediment flux and river morphology downstream of the dams? Observations from aerial photographs were combined with detrital zircon (DZ) U-Pb geochronology to quantify the anthropogenic effect of dams on sand generation. DZ U-Pb were collected from modern river channel sands, modern point bar sands, and modern delta sand sample. The U-Pb age data were evaluated in the context of bedrock U-Pb age data from the Llano Uplift Grenville basement above the dams and DZ U-Pb age data from the 30-40 km wide outcrop belt of Paleocene Wilcox Fm., dominated by Mesozoic Western US zircons, below the dams to evaluate possible entrenchment and sediment generation below the dams. While the modern river sediment collected upstream of the dams are dominated by Grenville DZ U-Pb ages, point bars below the dams show an abrupt increase of Wilcox derived zircons with only 1/3 of zircons derived from above the dams. This appears to be at least in part due to significant re-incision and erosion downstream of the dams out of the Paleocene Wilcox Formation. The lack of significant sand bars for 33 river kilometers below the dams and the progressive increase in sand bar size further downstream, combined with new DZ U-Pb data suggest that the modern river is incising into the Paleocene Wilcox below the dams and generating new sand. This is corroborated by the presence of 35 Ma DZ farther downstream, incorporated from Oligocene units. This progressive entrenchment of the river below the dams and incorporation of DZ from stratigraphic units encountered downstream illustrates the short-term response of the river geomorphology and sediment generation in light of anthropogenic perturbations of the river. These data also have interesting implications for sediment budget of rivers and coastal processes after dam construction. Importantly, this study is a cautionary tale for the use of DZ U-Pb provenance studies of modern rivers and the impact of dams and anthropogenic perturbations, as the downstream DZ record does not appear to reflect the pre-dam provenance signature of the Colorado River of Texas.