Criterion approach to transverse drainages
Streams that incise across bedrock highlands such as anticlines, upwarps, cuestas, or horsts are termed transverse drainages. Their relevance today involves such diverse matters as highway and dam construction decisions, location of wildlife corridors, better-informed sediment budgets, and detailed studies into developmental histories of late Cenozoic landscapes. This dissertation outlines a new criterion-based approach to understanding transverse drainage incision. Individual characteristics and field evidence for each of the four known mechanisms to explain transverse drainages are catalogued from the literature and personal experience. The mechanisms include antecedence, superimposition, overflow, and piracy.
Analogue modeling efforts on a large stream table test the criteria's replicability. The experiments corroborate and add to the criteria for antecedence, overflow, and piracy. The stream table modeling failed to test superimposition. The updated criteria are then used to isolate the mechanism responsible for twenty randomly selected transverse drainages in the southwestern U.S.A. The breakdown for the twenty sites revealed eight overflow, seven superimposition, four antecedence, and one piracy. Individual proposed mechanisms at each site were classified on an ordinal scale of confidence and resulted in the following levels of confidence: five very low, five low, five moderate, three high, and two very high. Fifty percent of the studied sites received a rating of low or below, indicating substantial further work is required to improve our understanding of some transverse drainages. The required tests or fieldwork necessary to increase the ratings for each site stem easily from a criterion-based approach, and are outlined in this dissertation.
A regional examination of southwestern transverse drainages permits a tentative generalization of drainage development vis à vis the dominant extensional tectonic regime. The overall picture that emerges from this study is: (a) rivers that predate and persist through tectonic activity generate antecedent drainages; (b) high stream order drainages develop following tectonic activity and extend in a downstream direction through ponding and overflow, reestablishing through-flowing fluvial systems; and (c) the base-level lowering associated with overflow causes incision and the evacuation of accumulated fill deposits, often leading to the development of superimposed drainages in the breached basin.