Geomorphic Effects Research Articles

Agent-Based Modeling, Scientific Reproducibility, and Taphonomy: A Successful Model Implementation Case Study

There is a growing use of agent-based model (ABM) simulations to reconstruct past human-environment interactions. ABMs are useful in that they offer scientists the opportunity to model processes and study phenomena and systems that may not be otherwise reproducible or testable. Replication or re-implementation studies of ABMs are, however, infrequently undertaken, and there are few examples within archaeology or other social sciences. This paper documents the process of a successful ABM replication study, as well as two additional modifications to the original model. Results corroborate the findings of the original geoarchaeological model and indicate that episodic geomorphic events significantly affect archaeological deposit formation and the inferences drawn from associated radiocarbon records. One revision of the model further demonstrates that episodic fluvial events can create highly varied radiocarbon distributions. The second modification illustrates that excavation data helps to fill in hiatuses in radiocarbon chronologies on depositional landforms, although there is no effect across landscapes subject to erosion. This successful replication exercise also illustrates the value of Open Access data and analyses in reproducing, testing, and expanding upon archaeological research and theory building.

Three‐Dimensional Flow Structures and Morphodynamic Evolution of Microtidal Meandering Channels

AbstractThe planform evolution of tidal meanders is driven by interactions between channel morphology and periodically reversing tidal flows, which feed back into the development of erosional and depositional patterns. However, the paucity of quantitative data has so far undermined detailed analyses about the geomorphic effects of tidal flows within tidal meanders. Here we aim to bond the morphodynamic evolution of tidal meanders with the structure of three‐dimensional flow that shapes them. By means of an acoustic Doppler current profiler, we have surveyed the flow fields over three different tidal meandering channels in the salt marshes at San Felice (Venice lagoon, Italy), each characterized by distinct planform morphology and evolutionary dynamics. Mutually evasive paths followed by the maximum ebb and flood streamwise velocities determine periodic changes in both the position and the orientation of curvature‐induced cross‐stream flows. These secondary flows can be locally disrupted by patches of submerged vegetation, especially in meanders of small size, with direct implications for the morphodynamic evolution of meander bends. The latter is further affected by flow separation in sharp bends. Flow separation effectively reduces channel width, enhancing bank erosion due to increasing flow velocities. Moreover, it creates low‐velocity zones of recirculating flows at both the inner and outer banks, thereby promoting the formation of point bars and concave‐bank benches, respectively. By relating three‐dimensional flow structure to patterns of channel change, our results provide a first step to unravel the relation between flows and forms within tidal meanders, whose planform characteristics may differ greatly from their fluvial counterparts.

Sediment transport in a restored, river-influenced Pacific Northwest estuary

Predicting the success of future investments in coastal and estuarine ecosystem restorations is limited by scarce data quantifying sediment budgets and transport processes of prior restorations. This study provides detailed analyses of the hydrodynamics and sediment fluxes of a recently restored U.S. Pacific Northwest estuary, a 61 ha former agricultural area near the mouth of the Stillaguamish River in Washington, USA. Water level, flow velocity, and suspended-sediment concentration (SSC) were measured between 21 March 2014 and 1 June 2015 at breaches excavated in the former flood-protection levee to determine transport patterns and the net sediment budget of the restoration area. SSC within the restoration area was primarily controlled by SSC variability of the nearby main stem Stillaguamish, but coastal processes also played a major role in sediment delivery. Fluvial sediment loading was dominated by runoff events associated with rainfall that lasted hours to a few days. Additionally, the 22 March 2014 SR 530 (Oso) landslide elevated sediment supply to the restoration area and coastal region for several weeks, indicating the importance of distal geomorphic events to coastal sediment budgets in small mountainous river systems. Sediment fluxes were controlled by river SSC and tidal dynamics, which set the quantity of water transported into the restoration area. Peak water discharge at the restoration area was about 12% of the river discharge, and peak sediment flux at the restoration area was about 5% of the river sediment discharge, although net sediment import was <1% of the total river load. Although sediment was imported to the restoration area, and inferred rates of accretion appear sufficient to keep pace with present rates of local sea-level rise, full recovery is challenged by significant lost grade from historical subsidence and will likely take decades to centuries. These results have implications for estuary restoration planning globally and indicate the importance of understanding coupled fluvial–coastal processes.

Zircon 4He/3He fractional loss step-heating and characterization of parent nuclide distribution

The radiogenic ingrowth and diffusive loss of 4He in accessory minerals is used to determine the timing of low-temperature thermal processes such as shallow tectonic, magmatic, hydrothermal, and geomorphic events. While the thermal history experienced by a sample cannot be directly constrained by a single (UTh)/He total gas age, 4He concentration profiles, normalized against irradiation-induced spallogenic 3He, have been shown to reliably recover continuous thermal history constraints if the parent nuclide distribution is either spatially uniform or can be determined. As such, 4He/3He thermochronometry has been successfully applied to apatite. However, individual zircon grains can contain complex parent nuclide zonation. Using moderately zoned, euhedral zircon grains from the rapidly cooled, distal Fish Canyon Tuff, we investigated the potential of the zircon 4He/3He thermochronometer through combining 4He/3He diode laser fractional loss step-heating with detailed analysis of parent element zonation. We present a routine method for the quantitative characterization of parent nuclide zonation by LA-ICP-MS depth-profiling. This study also demonstrates the importance of thermal control and avoidance of intra-sample thermal gradients during laser step-heating fractional loss experiments. Accurate and stable thermal control was achieved with an optimized sample-holder design, laser defocusing, and PID control loops to minimize temperature overshoot, failure to reach set-point, and temperature drop-offs. These analytical developments illustrate both the pitfalls and promise of zircon 4He/3He thermochronometry and create a reproducible methodology for 4He/3He thermochronometric applications to zoned and slowly cooled zircon.

Geomorphic Effects of a Dammed Pleistocene Lake Formed by Landslides along the Upper Yellow River

In a previous study two pairs of paleo-landslides within an 8 km reach of the upper Yellow River were studied and dated back to ca. 80 ka, however the relationship between these two pairs of paleo-landslides were not explored. This study inferred that the initial pair of landslides (Dehenglong and Suozi) appearing contiguously and forming an upstream 46 km-long lake along the river may be triggered by earthquake events from nearby capable faults. Subsequently, backwater inundating the valley floor as the dammed lake formed may cause shear stress of sediments lowered on steep slopes adjacent to the River, and eventually induce the other two additional landslides (Xiazangtan and Kangyang) ~8 km upstream. This could be inferred from two optically stimulated luminescence (OSL) samples yielding ca. 80 ka also, which were collected from asymmetric folds 10 to 30 cm in amplitude within the bedding plane between lake/lakeshore sediment and landslide mass at the front lobes of the two additional landslides. We estimated the maximum volume of this dammed lake was 38 km3 and may generate an outburst flood with an estimated peak discharge of 6.1 × 105 m3/s, which may cause massive geomorphic effects and potential disasters upstream and downstream. It is important to better understand the geomorphic process of this damming event in mountainous area with respect to reflecting tectonic uplift, paleoclimatic change and forecast and mitigate hazards on the northeast Tibetan Plateau.

Geomorphic effects of gravel mining on coastal plain rivers in Mazandaran, Iran

Rivers in Iran are heavily influenced by human activities, so that they have undergone many changes. In the meantime, gravel mining is one of these activities that has several environmental impacts on the various aspects of river environments. The study area is located in the plain of Mazandaran Province in Iran. The concerned rivers originate from the northern slopes of the Alborz Mount and flow into the Caspian Sea. Most of the mountainous areas are covered with thicket forests, and in the plain section, agricultural lands and human habitats are the dominant land uses in the region. The geomorphic effects of sediment mining from river have been investigated in four aspects including river longitudinal profile and transverse profile, as well as river pattern and changes in sediment size and amount. In Mazandaran Province, there are more than 60 rivers in which there are more than 80 active gravel mines. These mines are run by governmental, cooperative and private sectors. Currently, the dependence of the construction and development activities of the province on these gravel resources has seriously threatened the river bed with excessive exploitation, so that these exploitations would be of the most destructive activities imposed on the aquatic ecosystems. The results also show that the unauthorized sediments removal from the river bed and the construction of engineering structures to control its geomorphic consequences only delay the performance of river processes and, consequently, delay the morphological adjustments.

Drainage and erosion of Cambodia’s great lake in the middle-late Holocene: The combined role of climatic drying, base-level fall and river capture

We provide evidence for a large-scale geomorphic event in Cambodia’s great lake, the Tonlé Sap, during the middle Holocene. The present-day hydrology of the basin is dominated by an annual flood pulse where water from the Mekong River raises the lake level by c. 8 m during the monsoon season. We present new subsurface geophysical data, allied to new and past core studies, which unequivocally show a period of major mid-Holocene erosion across the entire Tonlé Sap basin that is coincident with establishment of the lake’s flood pulse. We argue that this widespread erosion, which removed at least 1.2 m of sediment across the lake’s extent, was triggered by up to three, likely interacting, processes: (1) base-level lowering due to mid-Holocene sea-level fall, leading to (2) capture of the Tonlé Sap drainage by the Mekong River, and (3) a drying climate that also reduced lake level. Longer-term landscape evolution was thus punctuated by a rapid, river capture- and base-level fall- induced, lake drainage that established the ecosystem that flourishes today. The scale of change induced by this mid-Holocene river capture event demonstrates the susceptibility of the Tonlé Sap lake to ongoing changes in local base-level and hydrology induced by anthropogenic activity, such as damming and sand mining, within the Mekong River Basin.

Методы дендроиндикации экзогенных гравитационных процессов: обзор

Since the beginning of the 21st century dendrogeomorphological research are abruptly activated in Western Europe but are appreciable reduced in Russia. In the present foreign literature, the new methods and approaches of dendrogeomorphological research has been developed and discussed. The article goal is an overview of the dendrogeomorphological methods for identifying, dating and reconstructing the geomorphic events (debris flows, avalanches, rockfalls), their advantages and limitations at different stages of a dendro-geomorphological study. The dendrogeomorphological research are based on the J. Shroder’s “process— event-response” concept. The disputable questions and unsolved issues of the dendrogeomorphology are considered.

Geomorphic changes in the Jhelum River due to an extreme flood event: a case study

The unprecedented extreme flood occurred in Jhelum basin, India in the first week of September, 2014 due to continuous spell of high intensity antecedent rainfall. The rainstorm observed over the basin during the event was unprecedented in terms of accumulated rainfall. In this study, Operational Land Imager (OLI) Landsat images taken before and after flood were employed to assess the morphologic and geomorphic effects of the flood along both the sides of the Jhelum River. The total erosion and sedimentation for the study reach due to flooding were 278 and 2.7 ha respectively. The flow length and the average channel width were decreased by 500 m and 25 m respectively. The study reach was divided into meander loops (M), straight stretches (S), and island bars (B) based on sinuosity and braiding. The maximum erosion in the meander loops was observed in M5 section, B4 section in island bars, and S7 section in straight stretches of the river. The maximum erosion was observed in the riparian cover due to this flood. However, damage to the built-up cover was practically very limited to the selected river reaches due to the reinforced channel banks. It is evident from the findings of the erosion on different sections of the meander loops and straight reaches that as the sinuosity increases, the erosion also increases for a relatively significant areal extend of each land cover. This result gives insight to the incompetent management measures of riparian cover, which is a principal reason for the huge economic losses during flood.

Small-scale lobate hillslope features on Mars: A comparative 3D morphological study with terrestrial solifluction lobes and zebra stripe lobes

Small-scale lobes (SSL) observed at the surface of Mars are thought to be the result of solifluction. Hence, their existence has important implications for our understanding of recent climate history, the distribution of thaw liquids and its geomorphic effects, as water is believed not to have been recently stable at the martian surface. These small-scale lobes are limited to sloping terrains and often occur alongside polygonal patterned ground attesting to the presence of ice in the environment. In this study, we perform a comparative 3D morphometric analysis of the lobes on Mars with terrestrial analogues. We use data from High Resolution Imaging Science Experiment (HiRISE) camera in order to create Digital Terrain Models (DTMs) on Mars at 1 m/pix. For the terrestrial analogues, we use DTMs and orthophotos from five periglacial environments with solifluction lobes and from one site with lobate features found in the Atacama Desert. The lobate features in the Atacama Desert are found within features called “zebra stripes”, which are apparently unique to the Atacama and are now generally accepted to be the result of seismic shaking. We show that SSL on Mars overlap morphometrically with terrestrial solifluction lobes, and are less similar to zebra stripe lobes. The majority of martian SSL occur below the angle of repose which demonstrates that a dry fall mechanism cannot be responsible for their formation and that lubrication of the subsurface is required. We found that martian SSL only occur on slopes greater than ~10°, a constraint not observed for terrestrial solifluction lobes nor zebra stripe lobes in Atacama. We hypothesise that this apparent constraint does not result from the lowered gravity on Mars, but instead from differing soil properties and/or thermal regime between Mars and Earth which affects the slope at which creep can initiate. We cannot rule out a role of CO2 sublimation in lobe-formation based on our terrestrial measurements.

Quantifying the habitat and zoogeomorphic capabilities of spawning European barbelBarbus barbus, a lithophilous cyprinid

AbstractSuitable gravel availability is critical for the spawning success of lithophilous fishes, including redd builders. Redd construction during spawning can alter substrate characteristics, thereby influencing hydraulic conditions and sediment transport, highlighting the importance of spawning as a zoogeomorphic activity. Here, interactions between redd‐building fish and their spawning environment were investigated for European barbelBarbus barbuswith a comparative approach across three English rivers: Teme (western), Great Ouse (eastern) and Idle (central). Sediment characteristics of spawning habitats were similar across the rivers, including subsurface fine sediment (&lt;2 mm) content (≈20% dry weight), but elevated subsurface silt content and coarser surface sediments were found in the river Teme. Water velocities were similar at spawning sites despite differences in channel width and depth. Redds were characterized by a pit and tailspill, with no differences in surface grain‐size characteristics between these and the surrounding riverbed, but with topographic alteration (dimensions and tailspill amplitude) in line with those of salmonids. Estimates of the fraction of the bed that spawning barbel were capable of moving exceeded 97% in all rivers. Estimated reproductive potential varied significantly between the rivers Idle and Teme (3,098 to 9,715 eggs/m2), which was largely due to differences in barbel lengths affecting fecundity. Larger barbel, capable of producing and depositing more eggs, but in more spatially extensive redds, meaning fewer redds per given surface area of riverbed. Predictions of barbel egg mortality based on sand content were low across both rivers. The effects of silt on barbel egg and larvae development are unknown, but the levels detected here would significantly impact salmon egg mortality. Similarities in fish length to redd area and the size of moveable grains by spawning barbel and salmon suggest they have similar geomorphic effects on sediments, although fine sediment tolerance is highly divergent.

The effect of sand fencing on the morphology of natural dune systems

Sand fences are a frequently used management tool on developed coastlines because they are inexpensive and easy to install. While the geomorphic effect of sand fences has been investigated before, previous studies have been limited in both temporal and spatial domains. Here, we present the evolution of Bogue Banks, a developed barrier island along the Outer Banks of North Carolina, over a 20-year period from 1997 to 2016 where sand fences were emplaced along parts of the island in 2010. We use LiDAR-derived cross-shore transects (n = 38,454) to measure beach and foredune features along the extent (~40 km) of Bogue Banks for every available year of lidar data as well as the locations of sand fences, which we identify in 39% of the transects following fence construction. First, we found that vertical growth of the natural foredunes along Bogue Banks was slightly positive between 2010 and 2016 despite an increased amount of shoreline erosion. This pattern was coincident with a combination of sand fence installation and beach nourishment efforts, which were most heavily focused on the eastern end of the island. Second, we found that natural foredunes located behind fenced dunes are typically shorter, wider, and smaller in volume than natural foredunes in non-fenced and undeveloped areas. Although this may partly be due to a tendency for fences to be installed in front of more vulnerable dunes, our results suggest that, once emplaced, sand fences prevent growth of the landward foredune behind fenced dunes. These findings suggest that sand fences block sediment supply to landward dunes, leading to a shorter and wider complex foredune than would otherwise naturally occur.

Geomorphology of the Anthropocene in Mediterranean urban areas

Urban-geomorphology studies in historical cities provide a significant contribution towards the broad definition of the Anthropocene, perhaps even including its consideration as a new unit of geological time. Specific methodological approaches to recognize and map landforms in urban environments, where human-induced geomorphic processes have often overcome the natural ones, are proposed. This paper reports the results from, and comparison of, studies conducted in coastal historical cities facing the core of the Mediterranean Sea – that is, Genoa, Rome, Naples, Palermo (Italy) and Patras (Greece). Their settlements were facilitated by similar climatic and geographical contexts, with high grounds functional for defence, as well as by the availability of rocks useful as construction materials, which were excavated both in opencast and underground quarries. Over centuries, urbanization has also required the levelling of relief, which was performed by the excavation of heights, filling of depressions and by slope terracing. Consequently, highly modified hydrographic networks, whose streams were dammed, diverted, modified in a culvert or simply buried, characterize the selected cities. Their urban growth, which has been driven by maritime commercial activities, has determined anthropogenic coastal progradation through port and defence or waterfront works. Aggradation of artificial ground has also occurred as a consequence of repeated destruction because of both human and natural events, and subsequent reconstruction even over ruins, buried depressions and shallow cavities. As a result, the selected cities represent anthropogenic landscapes that have been predominately shaped by several human-driven processes, sometimes over centuries. Each landform represents the current result, often from multiple activities with opposing geomorphic effects. Beyond academic progress, we believe that detecting and mapping these landforms and processes should be compulsory, even in risk-assessment urban planning, because of the increase of both hazards and vulnerability as a result of climate-change-induced extreme events and extensive urbanization, respectively.

Disentangling effects of forest harvest on long-term hydrologic and sediment dynamics, western Cascades, Oregon

The magnitude of sediment yield following forest timber harvest is controlled by increases in both sediment supply and streamflow. Since the relation between sediment transport and streamflow typically follows a power law, small increases in streamflow may translate into large increases in sediment transport. Interpreting the geomorphic effects of streamflow increases is confounded by the fact that timber harvest influences both the hydrologic regime and sediment supply of a watershed simultaneously, making it difficult to isolate the streamflow effect alone. Here we report on a novel approach to this problem using long-term data from two paired catchments located in the H.J. Andrews Experimental Forest, Oregon, USA. We use observed streamflow from the treated (clearcut) and control watersheds to reconstruct a natural streamflow time series for the treated watershed, one that represents streamflow response in conditions prior to harvest. We combine this reconstructed natural streamflow time series with observed relations between streamflow and sediment transport to quantify the background sediment yield and disentangle the relative effects of changes in hydrology and sediment supply. Results indicate that while increases in streamflow can account for modest increases in sediment transport, this is dwarfed by the increased supply of sediment that accompanies most timber harvest. These results have broad relevance to forest timber harvest and fuel management practices worldwide and can be used to constrain or set bounds on likely effects of more modest (i.e., thinning) techniques.

Middle Pleistocene formation of the Rio Grande Gorge, San Luis Valley, south-central Colorado and north-central New Mexico, USA: Process, timing, and downstream implications

The Rio Grande is the fourth longest river in North America extending over 3,000 km from the Rocky Mountains to the Gulf of Mexico. The Pleistocene evolution of this river from individual subbasins into a coalesced fluvial system has been long debated. Herein, we constrain the middle Pleistocene evolution of the northernmost and largest Rio Grande basin, the San Luis basin, and the timing of incision of the Rio Grande Gorge, based on new geologic mapping, 3He surface exposure dating, and U-series dating on pedogenic carbonates. 3He dating of shoreline deposits of closed basin Lake Alamosa and fluvially scoured bedrock along the Rio Grande gorge rim indicate the San Luis basin was connected with southerly Rio Grande basins and progressively incised since ∼400 ka. Waning tectonic activity, coupled with the hydrologic response to intensified middle Pleistocene glaciation, drove southward spillover of Lake Alamosa ∼400 ka, expanding the river's drainage basin by nearly 24,000 km2, and adding recharge areas in the high-altitude, glaciated San Juan, Tusas, and Sangre de Cristo Mountains. Geologic barriers to streamflow on the Taos Plateau were breached and basin tributary drainages were integrated and inset into their present canyons ∼250-200 ka, with approximately 120 m of incision during ∼400-200 ka. Previous work demonstrates that southerly streamflow through basins south of the SLB terminated into a large, rift-related bolson complex until the middle Pleistocene, and most axial Rio Grande river incision has occurred since 630 ka. We propose that middle Pleistocene integration of the SLB was the major geomorphic event that eroded Plio-Pleistocene tectonic and volcanic barriers and re-established flow of the Rio Grande from the Rocky Mountains to the Gulf of Mexico.

Geomorphological imprints of episodic surface runoff in deserts

Flash floods in desert, however they are rare, have significant geomorphic effects. The present paper reports the 4th November 2015 flash flood that occurred in the Western Desert of Egypt. Satellite images processing, field survey, close range photogrammetry and sediment analyses allow the quantification of flood geomorphologic impacts at different size scales. At Wadi An-Natrun depression, lake size doubled, hundreds of new ephemeral ponds appear. All ponds were dry by the next summer and lake margins were transformed into salt pans. At a local scale, four detailed case studies show how much geomorphic impacts of the same flash flood event vary in terms of gully morphology, sediment failure or stability, sediment size transport from clays to blocks and fan extension. This variability depends on local soil erodibility, including bedrock heterogeneity, soil cohesion and permeability, vegetation cover and inherited topography. Organic muds formed into ephemeral ponds and playa may provide a relevant sedimentary proxy for past flood events in the Western Desert of Egypt.

Formation and evolution of the Holocene massive landslide-dammed lakes in the Jishixia Gorges along the upper Yellow River: No relation to China's Great Flood and the Xia Dynasty

The massive Holocene landslide-damming in the Jishixia Gorges along the upper Yellow River was previously suggested to be related to China's Great Flood at 4.20–4.00 ka BP and the initiation of the first dynasty in China: the Xia Dynasty. Here, we present seven sedimentary profiles in the Xunhua Basin and Jishixia Gorges. The lithological, sedimentary and chronological results, including optically stimulated luminescence (OSL) and AMS 14C ages, indicate that there were two separate landslide-dammed lakes rather than one during the Holocene in the Jishixia Gorges. The first lake relates to the Maerpo landslide that occurred at ca. 8.30 ka BP. This lake gradually shallowed and then disappeared by ca. 6.30 ka BP as the landslide dam was slowly incised by the Yellow River. The second Gelongbu landslide occurred at ca. 6.10 ka BP. The consequent lake also gradually shallowed and then disappeared at ca. 5.60 ka BP as the landslide dam was cut through by the Yellow River. Both landslides and their associated lakes preceded The Great Flood in the middle-lower Yellow River by at least 1400 years and preceded the beginning of Xia Dynasty by at least 1600 years. The geomorphic events and therefore unrelated to these historical events.

Geomorphic complexity and sensitivity in channels to fire and floods in mountain catchments

Fires and floods are important drivers of geomorphic change. While the hydrologic and geomorphic effects of fires have been studied at the hillslope scale, we have much more limited data on post-fire runoff, channel changes, and inferred or measured sediment storage and delivery at larger scales. In this study we intensively documented channel changes over four years in two very similar ∼15 km2 catchments in the northern Colorado Front Range, Skin Gulch and Hill Gulch, that were burned primarily at high and moderate severity in June 2012. Ten and 11 cross sections and longitudinal profiles along the lower channel network were repeatedly surveyed in Skin Gulch and Hill Gulch, respectively. Summer thunderstorms generally caused deposition in the valley bottoms while incision resulted from the intervening baseflows and spring snowmelt, but a very high intensity summer thunderstorm in Skin Gulch just one week after burning caused much more deposition than in Hill Gulch. Fifteen months after burning both watersheds experienced an exceptional, long-duration flood resulting from a multi-day rainstorm with a several hundred year recurrence interval. This removed nearly all of the deposited post-fire sediment along with some of the older valley bottom deposits. The expansion and coarsening of the channel has greatly decreased the geomorphic sensitivity of both watersheds to future floods, and eliminated the more typical and persistent post-fire depositional signature. The channel changes in Skin Gulch after the fire and long-duration flood were much greater than in Hill Gulch, and this can be attributed to the much greater sediment deposition in Skin Gulch shortly after the fire, reduced geomorphic sensitivity in Hill Gulch resulting from a large erosional flood in 1976, and the spatial distribution of burn severity and storm rainfalls leading to lower peak flows in Hill Gulch. These results suggest that fires in the Rocky Mountains can trigger significant and dynamic hillslope and channel changes over sub-decadal timescales, but unusually long or intense rainstorms can cause larger and more persistent changes regardless of whether a catchment has recently burned.

If a tree falls in an urban stream, does it stick around? Mobility, characteristics, and geomorphic influence of large wood in urban streams in northeastern Ohio, USA

Large wood influences geomorphological, ecological, and biogeochemical functions of streams, but in urban areas it is also considered an unaesthetic hazard. Quantifying the abundance and mobility of in-stream wood along an urban intensity gradient, and understanding the relationship between wood dynamics and stream reach, network, and watershed characteristics allows for a stronger understanding of the potential benefits and hazards of wood in urban streams. Repeat wood surveys were conducted at 11 stream reaches in the Cleveland, Ohio area, where wood removal is not practiced and forested riparian zones exist. Study reaches span a gradient of impervious surface cover and extent of the stream network with a forest buffer. Channel morphology and sediment size were also measured. Stream hydrology does not change when imperviousness is 12–40% in this region, so quantification of impervious surfaces stands in for a broader suite of urban impacts.Wood abundance decreases – and mobility increases – as urban intensity in the watershed and along the stream network increases. Jams become smaller and less stable, but the size and orientation of individual wood pieces do not change along an urban gradient. Over the 6-month study period, wood mobility exceeded 100% in some of the most urbanized streams and transport distances are inferred to be on the order of 100s of meters. There was a positive correlation between jam spacing and pool spacing, and finer sediment sizes occurred in some wood-associated deposits, but overall geomorphic effects are limited. Where reaches are confined, wood abundance is suppressed, but impervious surfaces and the extent of forest along the stream network are stronger predictors of wood dynamics than reach scale characteristics. These results emphasize the importance of situating understanding of reach-scale wood dynamics in the context of larger scale phenomena.

Geomorphic Responses to Extreme Rainfall, Catastrophic Flooding, and Dam Failures across an Urban to Rural Landscape

The extreme rainfall of October 2015 in South Carolina generated numerous dam failures and spawned the flood of record at most U.S. Geological Survey stream gauges. Detailed field sampling and systematic image analysis are used to document the immediate and sustained geomorphic adjustments at four failed dams within the urbanized Gills Creek watershed. That urban focus is augmented with a similar analysis at five failed dams in more rural settings where less urban infrastructure exists. We also document the magnitude and type of geomorphic adjustments throughout the Gills Creek watershed unrelated to dam failures. Despite the extreme rainfall and associated flooding, the geomorphic effects were limited and localized, manifesting primarily at dam failures but not progressing significantly downstream, especially along Gills Creek, where the combination of intact dams, frequent urban roughness elements, thick floodplain vegetation, and numerous wetlands explains the lack of significant morphologic adjustments or major deposition. Dam failures in rural settings showed greater overbank deposition but the geomorphic effects were still limited to dam proximal locations because of the thick vegetal cover below the dam. Because these dams in rural and urban settings were constructed to maximize waterfront property, low gradient sites were selected; therefore, the geomorphic effects were unlike planned dam removals where massive headcutting, knickpoint migration, and sediment evacuation tend to occur. The most significant effect of the dam failures was the shift from an initial braided channel to a single thread channel within the emptied reservoirs, which occurred quickly, often within the first few months.