Transience of the North American High Plains landscape and its impact on surface water.

Ecosystem diversity and human activity in dry climates depend not just on the magnitude of rainfall, but also on the landscape's ability to retain water. This is illustrated dramatically in the High Plains of North America, where despite the semi-arid modern and past climate, the hydrologic conditions are diverse. Large rivers sourced in the Rocky Mountains cut through elevated plains that exhibit limited river drainage but widespread surface water in the form of ephemeral (seasonal) playa lakes1, as well as extensive groundwater hosted in the High Plains aquifer of the Ogallala formations2. Here we present a model, with supporting evidence, which shows that the High Plains landscape is currently in a transient state, in which the landscape has bifurcated into an older region with an inefficient river network and a younger, more efficient, river channel network that is progressively cannibalizing the older region. The older landscape represents the remnants of the Ogallala sediments that once covered the entirety of the High Plains, forming depositional fans that buried the pre-existing river network and effectively 'repaved' the High Plains3-6. Today we are witnessing the establishment of a new river network that is dissecting the landscape, capturing channels and eroding these sediment fans. Through quantitative analysis of the geometry of the river network, we show how network reorganization has resulted in a distinctive pattern of erosion, whereby the largest rivers have incised the older surface, removed fan heads near the Rocky Mountains and eroded the fan toes, but left portions of the central fan surface and the Ogallala sediments largely intact. These preserved fan surfaces have poor surface water drainage, and thus retain ephemeral water for wetlands and groundwater recharge. Our findings suggest that the surface hydrology and associated ecosystems are transient features on million-year timescales, and reflect the mode of landscape evolution.

Dynamic reorganization of river basins.

River networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels. We demonstrate that a characteristic metric of river network geometry gauges the horizontal motion of drainage divides. Assessing this metric throughout a landscape maps the dynamic states of entire river networks, revealing diverse conditions: Drainage divides in the Loess Plateau of China appear stationary; the young topography of Taiwan has migrating divides driving adjustment of major basins; and rivers draining the ancient landscape of the southeastern United States are reorganizing in response to escarpment retreat and coastal advance. The ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape-scale interactions among tectonics, erosion, and ecology.