MEGA-FLOOD EROSION OF THE CHANNELED SCABLAND

At the regional scale, major differences in the inferred erosional style and rate distinguish two Channeled Scabland erosional styles. One of these, exemplified by much of the Cheney-Palouse and Telford-Crab Creek scabland tracts, involved the invasion into well-developed (“mature”) stream and river valleys by massive, high-energy, mega-flood flows, with relatively little incision into the pre-flood valley floors. This style of erosion produced an “overfitness” relationship that contrasts markedly with other portions of the Channelecd Scabland, where massive vertical incision was achieved largely through the headward recession of pronounced sub-fluvial cataracts that migrated across drainage divides between pre-flood valleys. These scabland cataracts were not “waterfalls” in the conventional sense of that word because flow depths for the formative megaflooding were comparable to or even greater than the heights of the cararact headcuts. The cataracts varied in size and rate of formation with the energetics and timing of the formative mega-flood discharges. Examples include portions of both Grand and Moses Coulees, as well as Potholes, Frenchman, and East Lenore Coulees, as well a many smaller divide crossings, such as those containing Hudson, Trail Lake, and HU Ranch Coulees.

At local scales the erosive processes generated what J Harlen Bretz described as , “...a unique assemblage of erosional forms...an assemblage, which can be resolved into a genetic scheme only if time be very short, velocity be very high, and erosion chiefly by plucking of the jointed basalt” (1930, J. Geology, v. 38, p.422). Critical for the type of plucking envisioned by Bretz is the vertical lifting of joint-bounded basalt blocks by macroturbulent forces. This process cannot be the lateral sliding or toppling variety of so-called “plucking” that has been assumed for the recent theoretical modeling studies that have led to an advocating of “downsizing” for scabland-forming mega-flood discharges. These modeling studies have presumed sliding or toppling mechanisms that can be generated at much lower values of stream power intensity than would apply to the macroturbulent lifting processes required for the formation of rock basins and the narrow, deep mega-potholes that characterize Bretz’s scabland erosional landform “assemblage.”