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Features of landscape morphology—including slope, curvature, and drainage dissection—are important controls on runoff generation in upland landscapes. Over long timescales, runoff plays an essential role in shaping these same features through surface erosion. This feedback between erosion and runoff generation suggests that modeling long‐term landscape evolution together with dynamic runoff generation could provide insight into hydrological function. Here we examine the emergence of variable source area runoff generation in a new coupled hydro‐geomorphic model that accounts for water balance partitioning between surface flow, subsurface flow, and evapotranspiration as landscapes evolve over millions of years. We derive a minimal set of dimensionless numbers that provide insight into how hydrologic and geomorphic parameters together affect landscapes. Across the parameter space we investigated, model results collapsed to a single inverse relationship between the dimensionless relief and the ratio of catchment quickflow to discharge. Furthermore, we found an inverse relationship between the Hillslope number, which describes topographic relief relative to aquifer thickness, and the proportion of the landscape that was variably saturated. While the model generally produces fluvial topography visually similar to simpler landscape evolution models, certain parameter combinations produce wide valley bottom wetlands and non‐dendritic, trellis‐like drainage networks, which may reflect real conditions in some landscapes where aquifer gradients become decoupled from topography. With these results, we demonstrate the power of hydro‐geomorphic models for generating new insights into hydrological processes, and also suggest that subsurface hydrology may be integral for modeling aspects of long‐term landscape evolution. Plain Language Summary: The topography of landscapes affects how much and where precipitation becomes runoff, while runoff itself plays a role in shaping topography over long times through erosion. Some landscapes may exist in an equilibrium state, where the landscape is ideally shaped to carry the amount of runoff produced. Understanding this equilibrium may provide insights into why landscapes have different hydrological styles; for example, some landscapes contribute runoff to streams primarily through the ground, whereas others develop saturated areas during storms that generate surface runoff when rain falls on them. Here we use a new model to simulate dynamic runoff as we expect it to occur in humid temperate environments while also using this runoff to evolve topography. The results show that landscapes that already have a tendency to produce variably saturated areas because they are poor at storing water or transmitting it laterally through the ground also evolve to have lower relief, which helps variably saturated areas to persist. The results highlight the role that landscape history plays in the hydrological processes observed today and can be used to better understand the role of subsurface hydrological processes in long‐term landscape evolution. Key Points: A landscape evolution model with runoff from shallow groundwater was used to explore how hydrological function coevolves with topographyLandscapes evolve toward equilibrium where unchanneled uplands supply just enough for persistence of lowland saturated areasWe found local relief decreases log‐linearly with the fraction of runoff generated by saturation excess, in agreement with field studies [ABSTRACT FROM AUTHOR]

Whereas the existence of pronounced orbitally controlled periodicities is a major feature of Earth climate, its impact on landscape dynamics remains poorly understood. We use a Landscape Evolution Model (LEM) to systematically investigate the response of landscapes to a range of periodic oscillations in precipitation. The resulting sediment-flux evolution displays a pronounced sensitivity to the period of the input precipitation signal, such that, for a given erodibility, a specific periodicity maximizes the amplitude of the response. This optimal period of "resonance" scales as the inverse of the erodibility, but is progressively filtered out of the response when the intensity of hillslope diffusion increases. This frequency-dependent landscape behavior displayed by our model provides a mechanistic perspective on Molnar's (2004) proposition that ubiquitous changes in Late Cenozoic continental denudation could result directly from modifications in the spectral content of the climatic signal. © 2013. American Geophysical Union. All Rights Reserved.

River morphology and dynamics are strongly influenced by active tectonics. We report channel dynamics for the Peikang River, which flows through the Hsuehshan Range in central Taiwan. Using a digital elevation model and field surveys, we constrain channel morphology for an ~90 km stretch of river to calculate unit stream power and boundary shear stress along the river path. Incision rates are estimated with optically stimulated luminescence dating of sand deposited on strath terraces. We find a strong correlation between unit stream power/ shear stress and incision rate, but only if variation in channel width is considered. A calibrated river incision rule implies river incision rates of ~9-13.5 mm/yr upstream of the Meiyuan and Tili faults and suggests that one or both of these structures are presently active. Our results indicate that the Shuilikeng fault is also actively deforming, as incision rates increase to ~6-10 mm/yr across it, compared to 1-4 mm/yr in adjacent reaches. Prominent narrowing across the Shuilikeng fault, and the absence of significant gradient variation indicate that channel width is a first-order morphological adjustment to differential incision. Only when the channel width-to-depth ratio reaches a minimum does the channel slope significantly adjust to local changes in base level, as is the case upstream of the Meiyuan and Tili faults. doi: 10.1130/B30035.1

Earthquakes and bedrock river incision are fundamental processes in the evolution of tectonically active landscapes, yet little work has focused on understanding how a bedrock river responds to a single large earthquake. Data from the 1999 [M.sub.w] = 7.6 Chi-Chi earthquake in central Taiwan show dramatic differences in river response that depend on the proximity to the rupture zone. Near the fault scarp, vertical ground deformation intensifies river incision on a time scale of years to decades, while distal to the fault, landslides induced by the earthquake mantle bedrock on the river bed with sediment, impeding incision for decades to centuries as the material is evacuated. This surprising spatial and temporal variability in bedrock incision caused by earthquakes has implications for the paleoseismic interpretation of bedrock terraces and for the evolution of tectonically active landscapes. doi: 10.1130/G30883.1

We present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates. We show that traditional hydraulic scaling laws break down in this situation, and that channel widths become decoupled from drainage area upstream of the fault. Unit stream powers are ~4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled. The loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing. We show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault-induced adjustment of channel geometries has reset hillslope gradients. The results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first-order control the fluvial system exerts on the locus and magnitude of sediment supply to basins. Keywords: rivers, tectonics, faulting, channel geometry, geomorphology.

Incised ephemeral channels provide a window into the fluvial processes that help sculpt rangeland landscapes. This paper presents observations of ephemeral channels and valley networks in the high plains of Colorado, USA, with an eye toward painting a picture of the ingredients that must be included in mathematical models of landscape evolution in such environments. Channel incision in the study area is driven by summer thunderstorms, which can with reasonable frequency (3-5 yr) generate boundary shear stresses high enough to penetrate the highly resistant vegetation armor, but only within erosional hot spots where hydraulic forces are amplified by channel constriction and locally steep gradients. Focusing of erosion at these hot spots (which correspond to knickpoints and channel heads) is amplified by the small a real footprint and short 'erosional reach' of most convective storms. Upstream migration of knickpoints creates a pattern of short, active channel reaches separated by unchanneled or weakly channeled, fully vegetated stable reaches. Based on our observations, we interpret the necessary and sufficient conditions leading to the observed channel forms and dynamics as: (1) a resistant vegetation layer overlying an erodible substrate, which sets up a conditional instability through which erosional perturbations can grow by positive feedback; (2) high flow variability; (3) moderate to high substrate cohesion; and (4)a high volume fraction of fine-grained erodible material. Concave-upward valley long profiles are interpreted as a trade-off between downstream-increasing flood frequency and downstream-decreasing flood effectiveness. The observed process dynamics imply that long-term rates of valley incision should be especially sensitive to climatic oscillations between episodes of drought and warm-season convective rainfall. Keywords: arroyos, climate, erosion, gullies, networks, streams.