DO DOWNSTREAM PATTERNS IN BEDROCK VALLEY CONFINEMENT RECORD LANDSCAPE DISEQUILIBRIUM? (Invited Presentation)

Disequilibrium in bedrock landscapes in response to base level change or other perturbations can be long-lived. Bedrock channel knickpoints and fluvial terraces are commonly morphological features that record the legacy of bedrock landscape adjustment. More recently, studies have suggested that spatial patterns in bedrock valley morphology may also record these perturbations. Bedrock river valley morphology (e.g. valley width and valley relief) can record past climatic and tectonic forcing, as well as the relative influence of lateral erosion and vertical incision. A number of studies have demonstrated that bedrock valley aspect ratio at any individual cross-section may integrate the effects of lithology, tectonic uplift, climate, and sediment supply. Further, previous studies have indicated that for small catchments with relatively uniform lithology, bedrock valley width increases with drainage area following a general power-law relationship. Deviations from this downstream scaling have also recently been cited as signatures of disequilibrium introduced by drainage reorganization. However, the downstream variability of bedrock valley confinement index, which integrates measures of valley width with channel width and valley relief, have not been thoroughly explored.

In this contribution we aim to develop a null hypothesis to define expected downstream variability in bedrock valley confinement index under steady forcing scenarios. We demonstrate that under these conditions, bedrock valley confinement index does not scale monotonically with increasing drainage area, but rather, a range of complex spatial patterns in confinement index can emerge. We compare these null models with bedrock valley confinement indices extracted from the Upper Mississippi River Valley, where the history of perturbations due to past glaciations and drainage reorganizations is well-known. We demonstrate that downstream differences in channel confinement agree well with these known perturbations. We suggest that bedrock valley confinement may provide a promising holistic topographic measurement of landscape disequilibrium.