RECONSTRUCTING PALEOTOPOGRAPHY FROM RIVER NETWORKS WITH APPLICATION TO THE SOUTHERN SIERRA NEVADA

The relationships between different parts of river networks are quantifiable using a number of laws. Upstream parts of the network must have greater elevations than those downstream, confluences of two streams must share a common elevation, and slope and upstream drainage area are related to each other according to Flint’s law. Within this framework, parts of a river network, preserved as relict topography, strath terraces, or buried channel deposits, can be exploited to reconstruct the connecting drainage network and largescale paleotopography. This is achieved by integrating the slope-area relationship, allowing elevations to be predicted along the length of the river. Recasting the slope-area relationship as an inverse problem, we can use elevation constraints as data to infer model parameters defining local channel steepness and surface uplift. Here, surface uplift represents spatially variable tectonic processes that have either uplifted or buried the preserved topography. We highlight this approach with an example using relict topography across the uplifted and tilted southern Sierra Nevada. Here, surface uplift has produced fluvial knickpoints that propagate through the river network with relict topography preserved upstream of these knickpoints. The local channel slope information in individual reaches of the relict topography combined with the requirement of common elevations at confluences provides redundant information. Exploiting this redundant information provides maps of paleo-channel steepness and surface uplift. Surface uplift varies in space and supports the theory of regional, westward tilting. Importantly, it is only by exploiting information preserved across the entire relict river network using inverse methods that a tilting signal can be extracted from the slope of rivers. As the trunk streams of the major rivers draining the Sierra Nevada drain in the same direction as the inferred tilting signature, river slopes could represent two possible scenarios; lower-slope rivers that are regionally tilted or higher-slope rivers under spatially uniform uplift. It is only by incorporating slope information from tributaries that do not flow in the direction of inferred regional tilting signature that the pattern of surface uplift can be discerned.