How do rivers grow? Upstream controls on downstream landslide activity

Rivers grow by moving their drainage divides. Drainage divide migration can be (1) gradual, and widely distributed, such that each stream grows headward by similar amounts, or (2) rapid, and narrowly focused, such that a single stream grows at the expense of its neighbors and carves a gorge. Do rivers share the wealth? Or is most of the water and attention grabbed by just a few?

This talk describes patterns of drainage divide migration, downstream river incision, and bedrock landslide occurrence in two landscapes developed in gently-dipping sedimentary rock. First, we quantify drainage divide migration rates across a retreating escarpment within cuesta landscapes of southwestern Germany. Rates indicate that river growth is localized along mainstem rivers, which have reversed their flow direction and extended headward at rates ~10-1000 fold faster than long-term rock uplift and river incision rates. Growing mainstem rivers carve gorges into underlying stratigraphy, which includes erosionally-susceptible evaporites, marls, and swelling claystones, leading to a variety of slope failures and interesting geomorphic phenomena.

Halfway across the globe, a similar story is taking shape. Rivers of northwestern Arizona are growing and connecting across a series of sedimentary basins that were previously isolated by basin and range tectonism. Rivers integrate across these basins by a combination of filling, spilling, and headward erosion. As rivers grapple across water-stressed landscapes, the winners incise deeper into diverse packages of syntectonic basin stratigraphy. Landslide inventory mapping aided by newly available aerial lidar data and 1:24000 scale geologic mapping reveals associations between landslides and incision through swelling claystones deposited in these Neogene basins. Intersections between both factors and existing infrastructure are shown.

These case studies indicate that boundary conditions at the upstream end of river networks affect downstream hillslopes, prompting questions of how drainage divide migration, incision into underlying bedrock, and valley widening impact landslide activity.

 How do rivers grow? Upstream controls on downstream landslide activity, Neely (2026), USGS Landslide Hazards Seminar, 10 June 2026.