INCISION-RATE UNCERTAINTIES IN THE PRESENCE OF TRANSIENT AGGRADATION AND DEGRADATION OF DEBRIS FILL IN THE RIVER CHANNEL: THE COLORADO RIVER IN GRAND CANYON

Convexities in the longitudinal profile of the Colorado River in Grand Canyon span a wide range of wavelengths, from ≤ 1 km (the riffles and rapids) to ≥ 100 km, and at all wavelengths exhibit a strong correlation with the size and density of tributary debris fans along the river. It is our thesis that the long-wavelength convexities reflect the accumulation of tributary debris in the river channel, which we know to be the case for the short-wavelength convexities, the rapids. Debris-flow activity has increased significantly since the Holocene-Pleistocene transition due to the climate changes that attended it, and the river level (and river bed) appear to have risen by as much as tens of meters since that time. The several known episodes of late-Quaternary aggradation and subsequent incision of the Colorado River, with most of the Holocene devoted to its aggradation, suggest that transient debris-fill aggradation and degradation (incision) are integral parts of the recent geologic history of the Colorado River in Grand Canyon. Especially, then, for mid-to-late Quaternary incision-rate determinations, distinctions between “debris-fill incision” and “bedrock incision” are now in order. Using three different reference frames (channel bottom CB; river bottom RB; and river level RL), we illustrate these distinctions by examining transient aggradation/degradation of debris in simple triangular and rectangular river geometries for the case that vertical bedrock incision is zero (CB=0). Mostly because the subaqueous river/debris-fill/channel geometry is so poorly known, even at the present time, the distinctions between bedrock incision and debris-fill incision will not be easy to quantify. Moreover, any incision rate involving a depth of incision of the order of the (unknown) river/debris-fill/channel dimensions is subject to large uncertainties, of the order of the incision rate itself. Such must be the case for most mid-to-late Quaternary incision rates determined for the Colorado River in Grand Canyon.