HOLOCENE LANDSCAPE EVOLUTION IN BIG BEND NATIONAL PARK, SOUTHWESTERN TEXAS

Radiocarbon ages were used to define temporal and spatial patterns of Holocene erosion, alluviation, and landscape stability in the Rio Grande drainage network in Big Bend National Park. The arid to semiarid Big Bend region of southwestern Texas is in the northernmost part of the Chihuahuan Desert. Only one stream in the region, the Rio Grande, carries water throughout the year.

During the early and middle Holocene (ca. 10,000-4500 yr B.P.), alluviation in the Rio Grande valley was punctuated by floodplain stability, soil formation, and erosion. Buried soils dating to this period were documented in alluvium beneath the broad, flat, valley floor of the Rio Grande. These soils are deeply truncated, indicating that periods of pedogenesis were followed by episodes of erosion.

Between ca. 10,000 and 2000 yr B.P., tributaries in the Rio Grande drainage network were characterized by net transport of alluvium, with little or no long-term storage. It is likely that small drainage elements were actively downcutting, migrating laterally, and lengthening during this period. Sediment transported out of these streams accumulated on the valley floor of the Rio Grande or in alluvial fans. A major shift in the locus of sediment storage took place after ca. 2000 yr B.P., with tributaries becoming zones of net sediment storage. Late Holocene alluviation in the tributaries was punctuated by short episodes of floodplain stability and soil formation. Changes in temporal and spatial patterns of Holocene landscape evolution are attributed to major bioclimatic changes.