Accelerated floodplain sedimentation related to agricultural development of uplands has produced postsettlement alluvium (PSA) along rivers throughout the upper Midwest, U.S.A. Landscape characteristics, surficial sediments, and soils in the region vary geographically in relation to differences in geologic history, yet the extent to which this geographic variability influences PSA accumulation remains unexplored. This study uses existing data to assess how non-dimensional PSA thickness varies with landscape characteristics, surficial sediments, soils and climate. Geographic variability is associated with three subregions: 1) areas glaciated during the Late Wisconsin Episode (LWE), 2) areas glaciated during Pre-Illinois and Illinois Episodes (PI&IE), and 3) the Paleozoic Plateau (PP), an area where evidence of Quaternary glaciation is highly localized and does not influence geomorphic characteristics of the landscape. These subregions differ significantly in average geomorphic characteristics, including mean watershed slope (WS), mean local relief (LR), fraction of non-contributing area (NCA), pre-settlement drainage density (DD), and mean normalized river steepness (KSN). Native vegetation type also differs systematically between the subregions, creating significant differences in the frequency of alfisols (Alfi) and molisols (Mol). Thickness of last glacial loess (Loess) also varies across the region, although not systematically between the subregions identified. Non-dimensional PSA thickness differs significantly among the subregions, increasing systematically with landscape age, reflecting faster upland erosion rates and stronger connectivity of uplands to river corridors in older landscapes relative to more recently glaciated landscapes. Nondimensional PSA thickness is significantly positively correlated with LR, KSN, WS, Loess, Alfi, and Mol and significantly negatively correlated with NCA. Non-visibly distinct PSA is present in some LWE watersheds characterized by significantly lower KSN and WS relative to other LWE watersheds in which PSA is visibly distinct. PSA thickness and visibility reflect catchment-wide landscape characteristics and watershed-scale river steepness, which emphasize the importance of geographic setting, geological history, and landscape geomorphic characteristics for understanding historical river sediment dynamics. Spatial variability in PSA thickness also serves as an indicator of river system sensitivity to land-use change, providing insight into the relative impact of humans on rivers within different geographic settings.

Landscape dissection by rivers is a common qualitative measure of surface maturity. Quantitative studies of fluvial development over time indicate that drainage development increases non-linearly and is influenced by lithology, however, these studies typically take place over short timescales (10s of years), cover small areas, and focus on steep landscapes. In this work we use the Central Lowlands physiographic province (CL) as a natural laboratory in which we investigate rates and controls on drainage development in a post-glacial lowland landscape. Portions of the CL have been glaciated repeatedly in the Quaternary, and its topography is dominated by a patchwork of glacial landforms that have been developing drainage for 10 thousand to more than 500 thousand years. We modify the National Hydrography Dataset to estimate pre-agriculture drainage density developed over different amounts of time to reveal rates of drainage development in the CL. We find that drainage density in the CL increases non-linearly, increasing rapidly following glaciation before slowly approaching a maximum value. Much of the development is accomplished by 50 ka, well within a typical interglacial period. The apparent maximum value, ~1.5 km/km 2 , is comparable to the median drainage density measured in regions in the CL that have not experienced Quaternary glaciation. Our study shows that this value is likely influenced by soil sand content and regional precipitation levels. We note that while drainage density increases to an apparent maximum within an interglacial, the fluvial network is unlikely to adjust to post-glacial base level conditions within that same length of time. Our results are most consistent with a model of drainage expansion driven by the connection of closed depressions, or ‘non-contributing area’ (NCA), the portion of a watershed that does not drain to a river. We find that NCA decreases in tandem with increasing drainage density, which implies that NCA could be a measure of landscape integration that is at least as sensitive as drainage density.

Agricultural development has transformed the vegetation cover of many landscapes around the world, thereby altering water and sediment fluxes to river systems. Past work in the upper midwestern United States, particularly in areas of moderate relief, has shown that increases in water and sediment fluxes associated with agricultural development have dramatically altered river dynamics. Less is known about how agriculture has affected river dynamics, particularly rates of lateral migration, in relatively low relief landscapes of the Midwest shaped by glaciation during the Wisconsin Episode. This research examines rates of lateral migration of a channel bend along a lowland meandering river in Illinois, USA before and after agricultural development. The rate of lateral migration prior to agricultural development is estimated through dating of carbonaceous material within lateral-accretion deposits underlying distinct meander scrolls. The rate of lateral migration after agricultural development is determined from analysis of changes in river-channel position determined from survey records, aerial imagery, and digital elevation data. Average rates of migration before and after agricultural development are similar, suggesting that agricultural development has not substantially affected rates of lateral migration of the river. Some accelerated movement occurred locally following agricultural development, but this movement cannot be definitively tied to landscape transformation. Possible factors responsible for the lack of sensitivity of the river system to agricultural development include high resistance of the cohesive, tree-lined riverbanks to erosion and the low bankfull stream power per unit area of the modern river. From a management perspective, the study highlights the importance of bank vegetation in maintaining channel stability in low-relief agricultural landscapes.

Abstract. Glacier extent is known to be sensitive to climate variability through time. The impact of spatial variability in climate on glaciers has been much less studied. The Olympic Mountains of Washington State, USA, experience a pronounced precipitation gradient with modern annual precipitation ranging between ~6.5 meters on the high west-facing slopes to ~0.5 meters in the northeast lowlands. In the Quinault valley, on the west side of the range, a glacier extended onto the coastal plain reaching a maximum position during the early Wisconsin Episode glaciation. There is no evidence of a large Elwha glacier extending into the northeast lowlands at that time. We hypothesize that the asymmetry in past glacier extent was driven by spatial variability in precipitation. To evaluate this hypothesis, we first constrain the past precipitation gradient, and then model glacier extent. We explore variability in observed and modelled precipitation gradients over timescales from 6 hours to ~100 years. Across three data sets, basin-averaged precipitation in the Elwha is 54 % of that in the Quinault, with variability of less than 15 % at the annual timescale. Specifically, this ratio does not consistently vary with regional climate patterns. On average, modelled 6-hour accumulated precipitation in the Elwha is 78 % of that in the Quinault during a winter season, with a few low-precipitation time periods exhibiting a flatter or even reversed precipitation gradient. Overall, our analysis does not suggest a mechanism for increasing the precipitation gradient, but overwhelmingly indicates spatially coherent variability in precipitation across the peninsula. We conclude that the past precipitation gradient was likely similar to the modern gradient. We use a one-dimensional glacier flowline model, driven by sea-level summer temperature and annual precipitation to approximate glacier extent in the Quinault and Elwha basins. We find several equilibrium states for the Quinault glacier at the mapped maximum position within paleoclimate constraints for cooling and drying, relative to today. We assume the Elwha remained drier than the Quinault, and model Elwha extent for the climates of the Quinault equilibria. At the warm end of the paleoclimate constraint (10.5 °C), the Elwha remains a small valley glacier in the high headwaters. Yet, for the cooler end of the allowable paleoclimate (7 °C), the Elwha glacier advances to a narrow notch in the valley. As the ice is forced to flow through a smaller cross-section, it thickens, triggering an ice-elevation feedback. This feedback leads to rapid extension of the Elwha glacier to elevations only ~100 meters above those reached by the Quinault. While there is uncertainty in the glacial record of the Elwha, it is unlikely that such a large glacier existed during the most recent glaciation. Therefore, we suggest that the last glacial maximum climate was more likely to have been within the warm end of the paleoclimate range. Alternatively, spatially variable drivers of ablation including differences in cloudiness could have contributed to asymmetry in glacier extent. Future research to constrain past precipitation gradients and evaluate their impact on glacier dynamics is needed to better interpret the climatic significance of past glaciation and to predict future response of glaciers to climate change.

Glaciers are sensitive to temporal climate variability. Glacier sensitivity to spatial variability in climate has been much less studied. The Olympic Mountains of Washington state, USA, experience a pronounced orographic precipitation gradient, with modern annual precipitation ranging between ∼6500 and ∼500 mm water equivalent. In the Quinault valley, on the wet side of the range, a glacier extended onto the coastal plain, reaching a maximum position during the Early Wisconsin glaciation. On the dry side of the range, in the Elwha valley, there is no evidence of a large paleo-glacier during the Wisconsin glaciation. We hypothesize that asymmetry in the past glacier extent was driven by spatial variability in precipitation. To evaluate this hypothesis, we constrain the past precipitation gradient and model the glacier extent. We explore variability in observed and modeled precipitation gradients over timescales from 6 h to ∼100 yr. Across three datasets, basin-averaged precipitation in the Elwha is 54 % of that in the Quinault. Our analysis overwhelmingly indicates spatially coherent variability in precipitation across the peninsula. We conclude that the past precipitation gradient was likely similar to the modern gradient. We use a one-dimensional glacier flowline model, driven by sea level summer temperature and annual precipitation to approximate the glacier extent in the Quinault and Elwha valleys. We find several equilibrium states for the Quinault glacier at the mapped maximum position within paleoclimate constraints for cooling and drying, relative to present-day conditions. Assuming stable precipitation gradients, we model the Elwha glacier extent for the climates of these equilibria. At the warm end of the paleoclimate constraint (July average sea level temperature of 10.5 ∘C), a small valley glacier occurs in the high headwaters of the Elwha valley. Yet, for the cooler end of the allowable paleoclimate (July average sea level temperature of 7 ∘C), the Elwha glacier advances to a narrow notch in the valley, thickens, and rapidly extends far beyond the likely true maximum extent. Therefore, we suggest that the Early Wisconsin period was more likely to have been relatively warm because our models of the glacial extent are consistent with the past record of glaciation in both the Quinault valley and Elwha valley for warm conditions but inconsistent for cooler conditions. Alternatively, spatially variable drivers of ablation, including differences in cloudiness, could have contributed to past asymmetry in the glacier extent. Future research to constrain past precipitation gradients and evaluate their impact on glacier dynamics is needed to better interpret the climatic significance of past glaciation and to predict the future response of glaciers to climate change.

Complex interactions among water, dissolved and suspended material, and gases occur within the critical zone. These interactions depend upon and influence geologic and geomorphic processes, the chemical composition of constituents, and biological activities of microbes, higher organisms and associated ecological communities. All these components of the critical zone are co-evolving through inter-dependencies that extend over various space and time scales. In intensively managed agricultural landscapes, critical zone interactions are extensively disrupted to facilitate agro-ecosystem services. However, such disruptions are not evenly distributed across the landscape. Our research, conducted over eight years at the Intensively Managed Landscapes Critical Zone Observatory, demonstrates that the dynamics of intensively managed critical zones do not operate uniformly across time and space. Instead, critical interfaces, or zones of transition between different aspects of the landscape system, play a disproportionately important role in regulating material fluxes through mechanisms of storage, transport, and transformation, often through threshold responses and intermittent connectivity across these interfaces. We provide insight into how critical interfaces affect the intricate dynamics of water, energy, carbon, nutrients, and sediment in intensively managed landscapes. Since anthropogenic activities are continually and extensively modifying critical interfaces, sound understanding of the impact of these modifications is essential for intensive management to also be sustainable management.

In the low‐relief post‐glacial landscapes of the Central Lowlands of the United States, fluvial networks formed and expanded following deglaciation despite the low slopes and large fraction of the land surface occupied by closed depressions. Low relief topography allows for subtle surface water divides and increases the likelihood that groundwater divides do not coincide with surface water divides. We investigate how groundwater transfer across subtle surface water divides facilitates channel network expansion using a numerical model built on the Landlab platform. Our model simulates surface and subsurface water routing and fluvial erosion. We consider two end‐member scenarios for surface water routing, one in which surface water in closed depressions is forced to connect to basin outlets (routing) and one in which surface water in closed depressions is lost to evapotranspiration (no routing). Groundwater is modeled as fully saturated flow within a confined aquifer. Groundwater emerges as surface water where the landscape has eroded to a specified depth. We held the total water flux constant and varied the fraction of water introduced as groundwater versus precipitation. Channel growth is significantly faster in routing cases than no‐routing cases given identical groundwater fractions. In both routing and no‐routing cases, channel expansion is fastest when ~30% of the total water enters the system as groundwater. Groundwater contributions also produce distinctive morphology including steepened channel profiles below groundwater seeps. Groundwater head gradients evolve with topography and groundwater‐fed channels can grow more quickly than channels with larger surface water catchments. We conclude that rates of channel network growth in low‐relief post‐glacial areas are sensitive to groundwater contributions. More broadly, our findings suggest that landscape evolution models may benefit from more detailed representation of hydrologic processes.

During its 6,300-km course from the Tibetan Plateau to the ocean, the Yangtze River is joined by two large lakes: Dongting Lake and Poyang Lake. We explain why these lakes exist. Deglaciation forced the ocean adjacent to the Yangtze mouth to rise ∼120 m. This forced a wave of rising water surface elevation and concomitant bed aggradation upstream. While aggradation attenuated upstream, the low bed slope of the Middle-Lower Yangtze River (∼2 × 10−5near Wuhan) made it susceptible to sea level rise. The main stem, sourced at 5,054 m above sea level, had a substantial sediment load to “fight” against water surface level rise by means of bed aggradation. The tributaries of the Middle-Lower Yangtze have reliefs of approximately hundreds of meters, and did not have enough sediment supply to fill the tributary accommodation space created by main-stem aggradation. We show that the resulting tributary blockage likely gave rise to the lakes. We justify this using field data and numerical modeling, and derive a dimensionless number capturing the critical rate of water surface rise for blockage versus nonblockage.

Unlike well‐known plateaus associated with Cenozoic orogens, the Appalachian and Ozark Plateaus of the eastern United States fringe the foreland side of a long inactive and deeply eroded orogen. These foreland intracratonic plateaus (FIPs), which are underlain by sub‐horizontal cratonic‐platform strata and, in places, foreland‐basin strata, now lie 0.5–1.2 km above sea level, notably higher than adjacent fold‐thrust belts. An escarpment lies at or near the boundary between the FIPs and the fold‐thrust belts. Why did the topographic inversion leading to the development of the FIPs take place? To address this question, we built a numerical model, using Landlab, to simulate how topography evolves as foreland lithosphere flexes upward when post‐tectonic erosion causes unloading. In this model, flat‐lying cap‐rock strata (sandstone and limestone) of the foreland have greater resistance to erosion than do the deformed, tilted, cleaved, and fractured strata of the fold‐thrust belt, especially where the fold‐thrust belt contains argillaceous facies. We tested the model by characterizing the development of the Ozark Plateau in the foreland of the Ouachita fold‐thrust belt. Results demonstrate that regional isostatic uplift due to erosion, given reasonable differences in resistance to erosion between the fold‐thrust belt and the foreland, can generate the observed topographic inversion and a distinct escarpment, yielding a plateau. This model may help explain the post‐Paleozoic evolution of the Catskill Mountains, the Deep Valleys Province, and the Cumberland Plateau, highlands which border the Appalachian fold‐thrust belt.

The basal thermal regime of glaciers is a first-order control on the spatial patterns of glacial erosion. Polythermal glaciers contain both cold-based portions that protect bedrock from erosion and warm-based portions that actively erode bedrock. Climatic controls on the thermal structures of mountain glaciers and the spatial patterns of glacial erosion has received little study. In this study, we aim to fill this gap by modeling the impact of various climatic conditions on glacier basal thermal regimes and patterns of glacial erosion in mountainous regions. We couple a sliding-dependent glacial erosion model with the Parallel Ice Sheet Model (PISM) to simulate the evolution of the glacier basal thermal regime and glacial erosion in a synthetic landscape. We find that glacial erosion patterns follow the patterns of the basal thermal regime. Cold temperature leads to limited glacial erosion at high elevations due to cold-based conditions. Increasing precipitation could overcome the impact of cold temperature on the basal thermal regime by accumulating thick ice and lowering the melting point of ice at the base of glaciers. High precipitation rates, therefore, tend to cause warm-based conditions at high elevations, resulting in intensive erosion near the peak of the mountain range. Previous studies often assess the impact of climate on the spatial patterns of glacial erosion by integrating climatic conditions into the equilibrium line altitudes (ELAs) of glaciers, and glacial erosion is suggested to be maximal around the ELAs. However, our results show that different climatic conditions could produce glaciers with similar ELAs but different patterns of basal thermal regime and glacial erosion, suggesting that there might not be any direct correlation between ELAs and glacial erosion patterns.

Climate has been viewed as a primary control on the rates and patterns of glacial erosion, yet our understanding of the mechanisms by which climate influences glacial erosion is limited. We hypothesize that climate controls the patterns of glacial erosion by altering the basal thermal regime of glaciers. The basal thermal regime is a first-order control on the spatial patterns of glacial erosion. Polythermal glaciers contain both cold-based portions that protect bedrock from erosion and warm-based portions that actively erode bedrock. In this study, we model the impact of various climatic conditions on glacier basal thermal regimes and patterns of glacial erosion in mountainous regions. We couple a sliding-dependent glacial erosion model with the Parallel Ice Sheet Model (PISM) to simulate the evolution of the glacier basal thermal regime and glacial erosion in a synthetic landscape. We find that both basal thermal regimes and glacial erosion patterns are sensitive to climatic conditions, and glacial erosion patterns follow the patterns of the basal thermal regime. Cold temperature leads to limited glacial erosion at high elevations due to cold-based conditions. Increasing precipitation can overcome the impact of cold temperature on the basal thermal regime by accumulating thick ice and lowering the melting point of ice at the base of glaciers. High precipitation rates, therefore, tend to cause warm-based conditions at high elevations, resulting in intensive erosion near the peak of the mountain range. Previous studies often assessed the impact of climate on the spatial patterns of glacial erosion by integrating climatic conditions into the equilibrium line altitudes (ELAs) of glaciers, and glacial erosion is suggested to be maximal around the ELA. However, our results show that different climatic conditions produce glaciers with similar ELAs but different patterns of basal thermal regime and glacial erosion, suggesting that there might not be any direct correlation between ELAs and glacial erosion patterns.

Glacial erosion has shaped many mountain belts during the cold periods of the Late Cenozoic. The rate of glacial erosion is sensitive to the subglacial environment, including both the subglacial hydrology and the basal thermal regime. Geothermal heat from underlying bedrock is a major contributor to glacier energy budgets, controlling ice dynamics at the ice-bed interface by changing the basal temperature and the supply of meltwater. Despite the known influence of geothermal heat on the subglacial environment, its impact on glacial erosion has received little study. The geothermal heat flux in glaciated mountain ranges varies widely as a function of the tectonic setting. Therefore, if glacial erosion is sensitive to geothermal heat flux, the evolution of glaciated landscapes may depend upon tectonically-controlled geothermal gradients. We explore the impact of geothermal heat flux on the rates and spatial patterns of glacial erosion in mountain ranges using numerical models. We couple a sliding-dependent glacial erosion model with the Parallel Ice Sheet Model (PISM) to simulate the evolution of a synthetic glacial landscape. We find a robust tendency for increasing glacial erosion with increasing geothermal heat flux. The spatial pattern of erosion also varies with the magnitude of geothermal heat flux. At low geothermal heat flux, glacial erosion is consistently focused in major valleys. As geothermal heat flux increases, the area of significant glacial erosion expands into higher elevations and the rate of erosion increases. The location of maximum erosion migrates up-valley as geothermal heat flux increases, suggesting that glacial erosion tends to produce distinct landscapes as a function of geothermal heat flux. Our finding suggests that active mountain belts with high geothermal heat flux will express the glacial buzzsaw effect, in which high elevation topography is preferentially removed by glacial erosion. Glaciers at passive margins with low geothermal heat flux, in contrast, will tend to incise deep valleys at relatively low elevations. Previous work on the interaction between tectonics and landscape evolution has focused on relief generation and fracturing of rocks. Our results introduce a novel potential linkage between tectonics and erosion based on the sensitivity of glacial erosion to geothermal heat.