Knickpoint Propagation and Sediment Yield in an Experimental Steady State Drainage Basin: Size Matters

Can we detect knickpoint propagation in sediment yield records from eroding drainage basins? Knickpoint development and migration are common in experimental drainage basins experiencing constant rainfall and base level fall conditions, and such experiments offer a way of addressing this question. Previous experimental work has documented an increase in sediment flux following an abrupt drop in base level (Parker, 1977; Hancock, 1997). These experiments documented increase and exponential decay in sediment yield as the knickpoint propagated through the drainage basin. In the physical experiments presented here, a constantly dropping base level (equivalent to a continuous block uplift condition) forced the entire landscape to erode, and a balance between erosional flux and tectonic flux was maintained over longer times. Here, streams incised both by steady entrainment of bed material and by spontaneous knickpoint development and propagation. Imposed on the long term eroding landscape were self-generated epicycles of valley aggradation followed by knickpoint generation and stream incision. An automated measurement device recorded a detailed record of sediment flux leaving the basin. Time lapse video monitored knickpoint propagation. An analysis of the sediment flux record during several cycles of knickpoint propagation through the drainage network reveals that normal sediment flux oscillations mask the contribution of sediment flux due to knickpoint migration. Curiously, valley aggradation was not obvious in the sediment flux record either. Knickpoint heights for these experiments were about 0.5 cm. When scaled to maximum relief in the drainage basin, this height corresponds to a few percent of total relief in the basin. Some smearing of the knickpoint contribution occurs as sediment derived from knickpoint retreat is redeposited locally. Hence, no clear signal will be visible at downstream reaches. Only very large base level drops (greater than 1 cm, or about 5-10% of the basin relief) developed clear sediment flux signals. These results demonstrate that eroding stream networks exhibit significant short term variations in sediment transport and incision under steady forcing conditions, and such oscillations can hide the contribution from knickpoint migration.