Interpretation and downstream correlation of bedrock river terrace treads created from propagating knickpoints

Derivation of an analytical model for the slope of a strath terrace created following an upstream propagating wave of incision reveals that for detachment-limited river incision the exponent on channel slope, n, governs terrace tread slope. Terrace elevations can increase upstream (n > 1), downstream (n < 1), or can be horizontal (n = 1). Numerical modeling confirms these results for temporally evolving knickpoint geometries and for incision due to sudden base level fall and an increase in rock uplift rate. Except in the case of bedrock river incision with a slope threshold only exceeded during knickpoint propagation, a terrace created from transient headward incision contains no information about the paleo-channel gradient. Gradients in rock uplift rate along channels potentially complicate the interpretation of terraces by altering the primary tilt on strath terraces. In particular, monotonic gradients in rock uplift can produce terrace treads that are apparently folded. Because gradients in rock uplift rate are common, where terraces are not longitudinally traceable, care is warranted in terrace correlation. In simple tectonic settings where terraces are longitudinally traceable, the slope of a strath terrace created from headward incision may provide a means of estimating the dependency of river incision rate on channel slope. Terraces in four locations argued to have formed from headward incision are parallel or close to parallel with the active channel, implying a slope exponent on river incision rate that is much greater than one or a threshold slope for incision that is only exceeded during knickpoint propagation.