Control of Upstream Variables on Incised-Valley Dimension

Abstract

Abstract It is well documented that sea level fell during the Last Glacial Maximum, shifting graded-stream profiles out of equilibrium and causing rivers to incise into continental shelves. Although incised valleys have been heavily researched, the interplay between upstream and downstream controls on incised-valley dimension are not well constrained. To address this lack of understanding, we examined the cross-sectional dimension of nine incised valleys located across the northern Gulf of Mexico margin and bounded by the sequence boundary associated with the last sea-level lowstand. These incised valleys are distinguished by drainage basins that vary in size by three orders of magnitude, cover a margin that presently has a steep climate gradient, and extend across a continental shelf that varies along strike in width and gradient. Incision depths vary for valleys that have similar gradient profiles but different drainage-basin areas, suggesting significant control of upstream variables on incised-valley morphology. Additionally, these data show a strong linear correlation between drainage-basin area and incised-valley cross-sectional area. This suggests applicability of the empirically derived relationship between modern discharge and cross-sectional channel area to incised valleys when compared at the maximum highstand shoreline of the previous sequence. Incised-valley dimension adjusts over a longer period than the lowstand and is in equilibrium with drainage-basin area, which is considered a proxy for long-term discharge. Although base-level fall promotes incision, upstream variables control incised-valley dimensions.