PATTERNS OF TRANSIENT RIVER EROSION IN THE BOLIVIAN ANDES

We use a 50 m DEM to examine river profiles in the upper Beni River watershed, which drains the eastern flank of the Bolivian Andes.  There are a number of mapped folds and thrust faults throughout the network.  We hypothesize that channels will exhibit transient behavior where rock uplift rates were relatively large and where the channels have not yet had time to respond to a change in rock uplift rate.  We look for two indicators of transient response: (1) convex regions of the channel, or large knickpoints and (2) regions of the channel where the normalized channel steepness index (k_s) is relatively large, suggesting locally high erosion rates.  We map k_s throughout the entire network, and a number of patterns emerge.  In the central part of the watershed, almost all of the channels crossing a single mapped thrust fault are highly convex and have exceptionally large k_s values around the fault.  The only exception is the Consata River, which crosses the northern-most extent of the fault and has a smooth concave profile.  The Consata also has the largest drainage area where it crosses the fault, suggesting that there may be a drainage area threshold in the erosion process that has allowed the Consata to adjust more quickly to increased rock uplift rates than the smaller channels crossing this same fault.  Many of the tributaries that drain into the Consata upstream of the fault have large convexities, also supporting the idea that there may be a drainage area threshold in the transient channel response. We also identify transient channel behavior in channels draining an uplifted block in the southern part of the watershed.  Here, the upper reaches of these channels are concave with low k_s values, but where these channels drain off the block, they are convex and have extremely large k_s values, without exception.  However, a very large channel (drainage area > 5,800 km²) cuts straight through the block, and has a smooth, concave profile. Throughout the watershed, the trends in profile concavity and k_s values suggest that the transient response to rock uplift varies depending on drainage area.  The variation in k_s values also suggests that erosion rates have varied greatly throughout the upper Beni watershed.