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 Plains3,4,5,6. 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.
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We thank the Playa Lakes Joint Venture Project for data files on playa locations. We also thank J. K. Caves Rugenstein for discussions and suggestions, particularly with reference to birds. H.W.B. was supported in part by the National Aeronautics and Space Administration under grant NNX15AIO2H.
Nature thanks R. Duller and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Extended Data Fig. 1 χ map and χ versus elevation plots for modern-day South American megafans, constructed using a concavity of 0.45.
a, χ map, showing megafans formed in the Andean foreland of Bolivia and Argentina, with scaling as in Fig. 1. The Plicomayo River forms the northernmost prominent fan on the border of Bolivia and Argentina. The Juramento River forms the southernmost prominent fan. The Bermejo River forms the central fan. Points indicate channel heads of rivers highlighted in lower panels: B, channel heads for profiles shown in panel b; C, channel heads for profiles shown in panel c. Inset, map of South America, with the extent of the χ map outlined in red. b, Plot of χ versus elevation for the Plicomayo, Bermejo and Juramento river basins. Full sets of dendritic and fan-draining rivers define the black and grey envelopes, respectively; blue and red profiles show typical rivers from within each data cloud. c, Plot of χ versus elevation for the currently inactive portion of the fan.
Extended Data Fig. 2 Map of the river network in the Rocky Mountains and High Plains delineated by χ, calculated using a concavity of 0.3.
Patterns and major features do not differ greatly from those in the map constructed using a concavity of 0.45 (Fig. 1). The thick white line shows the extent of the Ogallala Group sediments. Thin white lines mark state boundaries. Labels denote river basins mentioned in the main text. River basins draining the Rocky Mountains are labelled near their headwaters; river basins draining fan surfaces covered in Ogallala sediment are labelled near their outlets. Coloured boxes show the locations of data provided in the indicated figures.
Extended Data Fig. 3 χ maps and χ versus elevation plots for the South Platte and Republican river basins.
a, χ map centred on the South Platte river basin, with the same colour scale as Fig. 1 (location shown in Extended Data Fig. 2). Points indicate the channel heads of rivers highlighted in the lower panels: S, South Platte River; R, Republican River. b, Plot of χ versus elevation for the South Platte river basin. Full sets of dendritic and fan-draining rivers define the black and grey envelopes, respectively; blue and red profiles show typical rivers from within each data cloud. The lower steepness tributary marked as the Gangplank drains a fan remnant that marks the farthest western extent of the Ogallala Group. c, χ/elevation plot for the Republican river basin. The main stems of dendritic rivers that largely drain the Rocky Mountains (blue lines) have large drainage areas and near-linear χ/elevation plots characteristic of an integrated river that is close to equilibrium. By contrast, rivers that drain surfaces capped by the Ogallala Group fan sediments show curved, divergent channel profiles (red lines), consistent with single-channel, non-branching rivers flowing directly down a planar surface.
Extended Data Fig. 4 Scaled distance maps and elevation profiles for the Canadian and Red river basins.
a, χ map centred on the Canadian river basin, with the same colour scale as Fig. 1 (location shown in Extended Data Fig. 2). Points indicate channel heads of rivers highlighted in the lower panels: C, Canadian River; R, Red River. b, χ-elevation plot for the Canadian river basin. c, χ-elevation plot for the Red river basin.
Plots were constructed using a range of concavity values, θ = m/n, from 0 to 0.6. The main stem of the Arkansas River is delineated with a solid black line. A concavity of 0.0 reduces χ to river distance from base level, so the upper left plot shows the longitudinal profiles of the river. We expect the main stems of large rivers draining the Rocky Mountains to be near equilibrium owing to their large drainage area and short response times. Slope/area analysis of the main stems of three of the major rivers (the Platte, Arkansas and Canadian rivers; Extended Data Fig. 7) indicates a concavity of about 0.3. With this concavity, the Arkansas River exhibits a concave-up, or kinked, form that is consistent with the harder, less erosive rock in the Rocky Mountains, and/or with the eastward tilt of the region.
These maps are centred on: a, the divide separating the Arkansas river basin from the Smoky Hill river basin; b, the divides separating the lower Canadian river basin from the North Canadian river basin in the north, and from the Red river basin in the south; and c, the divide separating the upper Colorado river basin and the upper Pecos river basin. See Extended Data Fig. 2 for names and locations. All three examples show that low relief areas occur where the fluvial network is poorly integrated, and that large changes in relief correspond to large cross-divide χ gradients. An exception is visible in panel b, centre left, where a large cross-divide χ gradient has no corresponding relief. This corresponds to a large river capture that transfers a tributary of the North Canadian to the lower Canadian River.
Extended Data Fig. 7 Slope/area regression in log space for the Platte, Arkansas and Canadian rivers.
Simultaneous regression for all three rivers yields a concavity, θ, of 0.3 and a steepness, ks, of 1.5.
Plots were constructed using a range of concavity values, θ = m/n, from 0 to 0.6. The main stem of the Republican River is delineated with a solid black line. The Republican river basin primarily drains fans that are capped by sediments of the Ogallala Group. The collapse of χ plots for m/n ≈ 0.0 and the curved, divergent channel profiles for m/n > 0.0 highlight that these rivers are single-channel, almost non-branching rivers flowing down an inclined planar surface.
Extended Data Fig. 9 Maps showing the distribution of wetland types on and off the Ogallala surface.
a, b, Maps centred on the eastern escarpments of the Red (a) and Brazos (b) Rivers. See Extended Data Fig. 2 for names and locations; see Methods for further details. The Ogallala surface is shown in light green and is covered in isolated wetlands (lakes and freshwater ponds). Away from the Ogallala surface, isolated wetlands are scarce: wetlands exist only along the river network, and are predominantly of the riverine and freshwater emergent and forested/shrub type.
a, Land cover. b, Mean annual precipitation, based on 30-year normal data for 1981 to 2010, downloaded from the PRISM Climate Group (http://prism.oregonstate.edu/normals/). The white outline delineates the Ogallala Group sediments. Contrasts in land cover are apparent between areas from which the Ogallala sediments have been eroded (now largely grassland) compared with where they remain (higher density of cultivated crops). For areas in and around the Arkansas river basin (black rectangle), 14% of land cover comprises cultivated crops, where the Ogallala sediments have been eroded; 58% comprises cultivated crops, where the Ogallala sediments are preserved (see Methods). A similar pattern of land use is present throughout the High Plains region. For the region east of and proximal to the Rocky Mountain front, and south of the North Platte river basin (where eolian deposits prevent cultivation), 12% of land cover comprises cultivated crops, where the Ogallala sediments have been removed; for 42% of land cover, the Ogallala sediments remain (Methods).
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Willett, S.D., McCoy, S.W. & Beeson, H.W. Transience of the North American High Plains landscape and its impact on surface water. Nature 561, 528–532 (2018). https://doi.org/10.1038/s41586-018-0532-1
- High Plains
- Dendritic Channel Networks
- Ogallala Formation
- Escarpment Retreat
- Conterminous United States
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