Hydrogeomorphic Controls Research Articles

Hydrogeomorphic controls on the thermal regime of natural and restored wetlands in a glacial plain

Wetland thermal regimes are underexplored but essential attributes of habitats and function for regulatory and action agencies to evaluate the success of nationwide restoration activities. Here we investigate regional and local controls on the hydrological and thermal regimes of small natural and restored wetlands in the St. Lawrence River Valley region of northern New York State, USA. We deployed pressure and temperature sensors in seventeen wetland ponds and adjacent groundwater piezometers to obtain hourly measurements of stage and water temperature for one year. For this humid continental climate, we observed seasonal hysteresis in air and water temperature, which extended to benthic and groundwater environments. We found the temporal behavior of wetland temperatures within this region were strongly similar across sites, suggesting two seasonal controls: regional atmospheric forcing as a primary control and local groundwater offset induced by hydrogeomorphic settings as a secondary control. In particular, wetland temperatures showed the greatest spatial variability with a consistent rank order among sites during summer (standard deviation ranging 0.9–2.8℃). Three regional hydrogeomorphic settings corresponded to varying thermal behaviors in dry summer due to different degrees of groundwater sourcing. Results demonstrate that the imprints of hydrogeomorphic controls on wetland thermal regimes are best assessed in both summer and winter, due to differences in groundwater availability of wetland thermal regimes.

Climatic and hydrogeomorphic controls on sediment characteristics in the southern Sierra Nevada

The magnitude of sediment yield from headwater catchments is controlled by the interactions among hydrology, geomorphology, and soil disturbance. In montane regions like the Sierra Nevada, snow is one of the main factors controlling the timing and magnitude of hydrological fluxes. However, the role of snow on modulating spatial and temporal variation of sediment yield remains unclear. Using 120-site years of sediment yield data from 10 headwater catchments (drainage area 50–475 ha), we examined the sediment yield characteristics across an elevational gradient (1,777–2,381 m elevation) in the southern Sierra Nevada. Across space and time, we calculated an average annual suspended sediment yield of 62 ± 147 Mg/km2. In contrast, the measured mean annual bedload yield from the study catchments was small, 1.1 ± 2.4 Mg/km2. A linear mixed-effects model showed that maximum annual discharge alone can only explain 24 % (marginal R2 = 0.24) of the variance in sediment yield. Similarly, the hypsometric integral, which is often used as a metric for erosion susceptibility, showed no predictive power (marginal R2 = 0.005). As much as 65 % of the variance in sediment yield can be explained by fixed effects when snow related drivers (i.e., center of flow timing and aspect) were included in the model along with maximum annual discharge, suggesting a strong influence of snowmelt. Moreover, the relationship between area normalized suspended sediment yield (Qs) and unit discharge (Q) was significantly different between rain and snow events (p = 0.001). Both average slope (α) and exponent (β) terms of the Qs=αQβ relationship across the ten catchments were higher for rainfall (α=5.8,β=1.84) than those for snowmelt (α=3.1,β=1.77) events. As the erosion severity and power were higher during rainfall than the snowmelt events, a shift in the precipitation form from snow to rain under a warming climate will likely increase sediment yield. These results provide critical insights on background sediment yield in the southern Sierra Nevada and likely changes under future climate.

Characterizing the trend of channel braiding of a tropical transboundary river using spatial growth component analysis and ARIMA model

Channel braiding is one of the fundamental fluvial processes in the context of sediment-energy nexus. The present study intends to simulate the channel braiding pattern of a tropical river (Jaldhaka River) on the Himalayan foreland plains. The grain size of the bed material in the Jaldhaka River decreases with distance from the mountain front, as does the surface gradient from the piedmont plains to the northern alluvial plains accompanied by the development of alluvial fans. Such assemblage results in multiple facets of hydro-geomorphic controls on channel planform dynamicity. This study is one of the pioneering attempts to characterize the spatial and temporal trend of channel braiding in any river running through the foreland region of the Sikkim-Bhutan Himalayas. The type of channel braiding is determined by the three indices – (1) Braiding Index (BI*), (2) Channel Count Index (BI), and (3) Channel Length Index (Pt), in cases where no gradual and uniform trend of development has been seen.The spatial allometric growth of the channel braiding and time series association using the ARIMA model was examined on the twenty stretches of the river. The results depict nonuniformity in planform development with distance downstream during a 30-year time span (1990–2020). The nature of the planform value distribution concerning all three applied indices has projected a polynomial trend where certain intermediate channel segments break the general trend of the braiding intensity with downstream distance. Since the downstream braiding intensity of the Jaldhaka River hasn't decreased either linearly or exponentially, we were unable to find any significant relationship of it with surface gradient. The nature and association of the channel braiding of the Jaldhaka River point towards the influence of local controls (e.g. river terracing on the upper part and faultline controls in the middle part) on channel planform dynamicity. The prediction of channel braiding for 2030 and 2050 coupled with the model fitted with RMSE, and BIC bears great significance in revealing the future bar growth which may be useful for the different planners and stakeholders concerned with the river basin development.

Cross-scale controls on the in-stream dynamics of nitrate and turbidity in semiarid agricultural waterway networks

Stream and riparian zone networks embedded in agricultural landscapes provide a potential intervention point to ameliorate the negative effects of agricultural runoff by reducing transport of nitrate (NO3−) and suspended sediments (SS) downstream. However, our ability to support and promote NO3− and SS attenuation is limited by our understanding of vegetative and hydrogeomorphic controls in realistic management contexts. In addition, agricultural landscapes are heterogenous on multiple management scales, from farm field to regional water management scales, and the effect of these heterogeneities and how they interact across scales to affect vegetative and hydrogeomorphic controls is poorly explored in many settings. This is especially true in irrigated agricultural settings, where stream and riparian networks are entwined with and sensitive to water management systems. To fill these gaps, we related the vegetative and hydrogeomorphic features of 67 waterway reaches across two water management districts in the California Central Valley to reach-scale NO3− and turbidity attenuation and district-scale water quality patterns. We found that in-stream NO3− attenuation was rare, but, when it did occur, it was promoted by shallow and wide riparian banks, low flows, and high channel-edge denitrification potential. Nitrate concentrations were consistently higher in upstream reaches compared to water district outlets, suggesting that while exports from the district were low, agricultural runoff may impair within-district water resources. Turbidity attenuation was highly variable and unrelated to vegetative or hydrogeomorphic features, suggesting that onfield controls are crucial to managing suspended sediments. We conclude that waterway networks have the potential to mitigate the effects of agricultural NO3− runoff in this setting, but that more effective monitoring and adoption of NO3− attenuating features is needed. Using our findings, we make specific management and monitoring recommendations at both reach and water district scales.

Patterns of Denitrification Potential in Tidal Freshwater Forested Wetlands

Limited evidence for spatial patterns of denitrification in tidal freshwater forested wetlands (TFFWs), seemingly due to high spatial variability in the process, is surprising considering the various spatial gradients of its biogeochemical and hydrogeomorphic controls in these ecosystems. Because certain physical environmental gradients may be useful for the prediction of denitrification in TFFWs, we measured denitrification and ecosystem attributes in hummock-hollow microtopography of TFFWs along longitudinal riverine positions (upper, middle, and lower tidal river sites, and nearby upstream nontidal forested floodplains) of the adjoining Pamunkey and Mattaponi Rivers, Virginia. We tested differences by river, site, and plot in denitrification enzyme activity (DEA) and substrate limitations of denitrification potential (DP). The Pamunkey River carries greater river nitrate concentrations, and we found less nitrate limitation of DP and greater soil nitrate in hollows of this river. DEA in tidal hummocks was positively correlated with soil organic matter, nitrogen, and carbon, with the highest rates in lower tidal sites. Hummocks also promoted greater oxygen-controlled substrate limitation of DP, whereby experimental aeration stimulated DP under subsequent inundation more in hummocks than hollows. Additionally, tidal sites had greater DEA than nontidal sites, inferred to be caused by a combination of higher moisture, organic, and nutrient content. Our results indicate that the increasing nitrogen concentrations in these rivers will increase denitrification more on the Mattaponi River by alleviating its greater nitrogen limitation compared to the Pamunkey River, and modification to sedimentation, inundation, or microtopography from sea level rise may alter denitrification gradients in TFFWs and upstream low-elevation nontidal floodplains.

Hydrogeomorphic controls on soil carbon composition in two classes of subalpine wetlands

Wetlands play a vital role in terrestrial carbon (C) sequestration, but the sensitivity of their C stocks to disturbance remains uncertain, requiring enhanced understanding of the processes that govern C storage and removal. The unique conditions in wetlands from different hydrogeomorphic (HGM) classes likely regulate the cycling, storage and vulnerabilities of wetland soil C stocks. To determine how differences in hydrogeomorphic setting influence soil organic carbon (SOC) processing, we compared C content and composition between depressional and slope wetlands located in the Colorado Rocky Mountains. Isolated depressional wetlands were characterized by seasonally declining water tables, slow discharge, high clay content, and thick organic horizons. Slope wetlands received perennial groundwater inputs and had coarser soil textures and thinner organic horizons. Seasonal snowmelt inputs coupled with low hydrologic discharge and higher clay content in depressional wetlands were predicted to sustain anoxic conditions, leading to high SOC content and chemically reduced C compounds. Depressional wetland soils had higher SOC content at depth and higher porewater DOC concentrations compared to slope wetland soils. Solid-state 13C nuclear magnetic resonance spectroscopy demonstrated that aliphatic compounds were the dominant SOC component in depressional wetlands compared to aromatic C forms in the slope wetlands. The higher prevalence of aliphatic carbon in depressional wetland soils suggests that stored SOC is protected by anaerobic conditions to a greater extent than in the slope wetlands, and that this SOC may be more vulnerable to drying and oxic conditions associated with wetland drainage and climate change.

Hydrogeomorphic controls on Holocene lacustrine loss-on-ignition records

Holocene loss-on-ignition (LOI) records were developed for radiocarbon-dated sediment cores from 20 lakes in the Uinta Mountains of northeastern Utah, USA. The hydrogeomorphic setting of each lake was characterized, permitting an analysis of connections between LOI values and lake setting. Median LOI values exhibit a significant negative correlation with lake elevation, with the range of elevations and the maximum slope of the watershed, with the extent of ice/snow and exposed rock in the watershed, and with the volume of through-flowing water. In contrast, LOI is positively correlated with the extent of forest cover and wetlands in the watershed. In addition, forward stepwise linear modeling demonstrates that lake area/lake depth ratio is a significant negative predictor for median LOI, that shoreline complexity is a significant positive predictor for maximum LOI, and the standard deviation of LOI values is greater in lakes surrounded by watersheds with more wetlands and surface water. Some LOI time series contain excursions at finer temporal scales that are recorded synchronously in adjacent lakes, however many excursions appear in only a single record. Similarly, step changes in LOI that are clear in some records are not preserved in the sediments of nearby lakes. Over longer timescales, some LOI records are essentially constant during the Holocene. In contrast, some lakes record steadily rising LOI values, and approximately half of the lakes have complicated LOI records with millennial-scale variability. Lakes with large surface area, large area/depth ratios, large watershed area/lake area ratios, rugged watersheds, and extensive ice/snow cover are more likely to record steady LOI values over time. The degree to which LOI records from neighboring lakes vary emphasizes that lacustrine proxy records reflect a strong filtering effect, whereby the signal of an external forcing is recorded to varying degrees depending on the nature of the lake and surrounding watershed.

Estuarine abandoned channel sedimentation rates record peak fluvial discharge magnitudes

Fluvial sediment deposits can provide useful records of integrated watershed expressions including flood event magnitudes. However, floodplain and estuarine sediment deposits evolve through the interaction of watershed/marine sediment supply and transport characteristics with the local depositional environment. Thus extraction of watershed scale signals depends upon accounting for local scale effects on sediment deposition rates and character. This study presents an examination of the balance of fluvial sediment dynamics and local scale hydro-geomorphic controls on alluviation of an abandoned channel in the Salinas River Lagoon, CA. A set of three sediment cores contained discrete flood deposits that corresponded to the largest flood events over the period of accretion from 1969 to 2007. Sedimentation rates scaled with peak flood discharge and event scale sediment flux, but were not influenced by longer scale hydro-meteorological activities such as annual precipitation and water yield. Furthermore, the particle size distributions of flood deposits showed no relationship to event magnitudes. Both the responsiveness of sedimentation and unresponsiveness of particle size distributions to hydro-sedimentological event magnitudes appear to be controlled by aspects of local geomorphology that influence the connectivity of the abandoned channel to the Salinas River mainstem. Well-developed upstream plug bar formation precluded the entrainment of coarser bedload into the abandoned channel, while Salinas River mouth conditions (open/closed) in conjunction with tidal and storm surge conditions may play a role in influencing the delivery of coarser suspended load fractions. Channel adjacent sediment deposition can be valuable records of hydro-meteorological and sedimentological regimes, but local depositional settings may dominate the character of short term (interdecadal) signatures.

Hydro-geomorphic controls on the development and distribution of acid sulfate soils in Central Java, Indonesia

Coastal planning policies and regulations in Indonesia have not adequately considered acid sulfate soils (ASS) as a constraint on development. Aquaculture is often undertaken in unsuitable areas, and fish and shrimp culture systems, in particular, fail after a short period of production because of the impacts of ASS on pond water quality and, subsequently, on fish and shrimp health. This study describes a mapping approach based on an understanding of the hydro-geomorphic controls on the formation and the distribution of ASS in Central Java, Indonesia. The underlying approach was to identify associations between ASS development and distribution within estuarine hydro-geomorphological units (HGUs). This study utilized a multi-level methodology involving multi-resolution, remotely-sensed data and GIS analysis, coupled with field and laboratory-based data, to obtain hydro-geomorphic and soil information at different mapping scales. An estuary classification scheme for Central Java identified river, tide and wave-dominated estuaries as the dominant estuary types. HGUs were identified in each estuary type to define the relationships between landform development processes and pyrite concentration in soil layers. Thirty-nine HGUs were classified based on landforms, marine and fluvial hydrology, geomorphic processes, land use, and vegetation types. Field and laboratory assessment of soil properties were undertaken to identify the horizontal distribution of ASS in the HGUS and its vertical character in soil profiles. In contrast to previous studies, the results showed that estuaries located in low-energy environments on the north coast have low (<0.1%) pyrite concentrations in 90% of their HGUs. Decades-old intensive aquaculture and dredging activities, that led to repeated oxidation and leaching, were identified as factors for low pyrite concentrations. On the south coast, the combination of a high river and marine energy environment created scattered landforms with soils that have very high (4–9%) pyrite concentrations. These ASS-bearing HGUs mostly developed in low- energy estuarine environments overlying former high river energy environments. The information generated for each HGU facilitated the development of an ASS mapping model that incorporates knowledge on the relationship between soil and landform formation in Central Java estuaries. Knowing where ASS occur is essential to minimize the risk of environment degradation. Using multi-resolution, remotely sensed data decreases cost and labour, compared to more traditional mapping approaches, especially to identify sampling sites for field surveys. The resulting maps and mapping methods will improve land capability assessment for brackishwater aquaculture and other coastal land use in Indonesia.

Hydro-geomorphic controls of greenhouse gas fluxes in riparian buffers of the White River watershed, IN (USA)

Riparian ecosystems are defined by the nature and regularity of the interactions between a given river system and its floodplains, and past studies have often presented vegetation cover as the exclusive expression of these interactions. There has been to our knowledge, no systematic attempt at linking greenhouse gases (GHG) fluxes and types of riparian buffers. The present study was conducted to investigate the intensity and seasonality of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes in riparian buffers in three common hydro-geomorphic settings (HGM) across the White River watershed (Indiana, USA). These classes included riparian sites located: (i) in till plain depressions near 1st order streams (HGM-1), (ii) in incised narrow valleys with thin alluvium layers above glacial till (HGM-2), and (iii) along 3rd–4th order streams in broad floodplains with thick alluvial and glacial outwash deposits (HGM-3). For each class, 3 sites were selected and GHG fluxes were measured during the wet (May) and dry seasons (August). Strong relationships were found between GHG fluxes, soil properties and environmental factors, but these relationships varied with season and gas species, making it challenging to rely on these relationships for GHG fluxes upscaling. Analysis of variance and discriminant analysis showed that the HGM-defined riparian buffers were distinct in terms of GHG flux intensity. Regardless of season, the HGM-1 sites emitted CO2 at rates 1.6 times higher than at the other sites, likely due to difference in soil C quality. During the wet season, N2O emission was significantly higher at the HGM-3 than at the other sites (0.88 vs 0.27mgNm−2d−1), and was negatively related with the gradient of the adjacent channel (r2: 0.69). The riparian buffers acted as CH4 sinks, with the HGM-2 sites exhibiting CH4 uptake rates significantly greater than the other riparian types (−0.80 vs −0.34mgCH4-Cm−2d−1). The consistency of these results underscores the potential of an HGM-based monitoring approach to derive watershed-scale GHG budgets for riparian buffers.

Hydrogeomorphic controls on hyporheic and riparian transport in two headwater mountain streams during base flow recession

AbstractSolute transport along riparian and hyporheic flow paths is broadly expected to respond to dynamic hydrologic forcing by streams, aquifers, and hillslopes. However, direct observation of these dynamic responses is lacking, as is the relative control of geologic setting as a control on responses to dynamic hydrologic forcing. We conducted a series of four stream solute tracer injections through base flow recession in each of two watersheds with contrasting valley morphology in the H.J. Andrews Experimental Forest, monitoring tracer concentrations in the stream and in a network of shallow riparian wells in each watershed. We found hyporheic mean arrival time, temporal variance, and fraction of stream water in the bedrock‐constrained valley bottom and near large roughness elements in the wider valley bottom were not variable with discharge, suggesting minimal control by hydrologic forcing. Conversely, we observed increases in mean arrival time and temporal variance and decreasing fraction stream water with decreasing discharge near the hillslopes in the wider valley bottom. This may indicate changes in stream discharge and valley bottom hydrology control transport in less constrained locations. We detail five hydrogeomorphic responses to base flow recession to explain observed spatial and temporal patterns in the interactions between streams and their valley bottoms. Models able to account for the transition from geologically dominated processes in the near‐stream subsurface to hydrologically dominated processes near the hillslope will be required to predict solute transport and fate in valley bottoms of headwater mountain streams.

A stochastic approach to modelling and understanding hillslope runoff connectivity dynamics

Runoff generation at the hillslope scale is an important component of the hydrological cycle. Recent work has shown that a common hillslope runoff response mechanism is driven by connectivity of saturated patches in the subsurface (via filling and spilling) to a threshold initiation of lateral flow at the hillslope base. Here, we show that directed percolation theory is able to represent this key runoff process including the details of dynamical flowpath development and filling and spilling processes at the soil-bedrock interface. We then use the directed percolation model to investigate how changes in slope angle, soil depth, and subsurface microtopography influence stormflow response. We map the evolving subsurface flow network under different hillslope classes and compare them to the natural system response. Our results suggest that the natural system sheds water more efficiently than randomly generated systems providing some insights into key hydrogeomorphic controls on water shedding in the environment.

A geophysical analysis of hydro-geomorphic controls within a headwater wetland in a granitic landscape, through ERI and IP

Abstract. Wetlands are undergoing considerable degradation in South Africa. As interventions are often technical and costly, there is a requirement to develop conceptual process models for these wetland systems so that rehabilitation attempts will be successful. This paper presents an approach using the geophysical methods of Electrical Resistivity Imaging (ERI) and Induced Polarization (IP) to delineate sub-surface hydro-geomorphic controls that maintain equilibrium disconnectivity of wetland-catchment processes, which through gully erosion are increasing the catchments connectivity through loss of water and sediment. The findings presented here give insight into the geomorphic processes that maintain the wetland in an un-degraded state, this allows for the development of a conceptual model outlining the wetland forming processes. The analysis suggests that sub-surface clay-plugs, within an otherwise sandy substrate are created by illuviation of clays from the surrounding hillslopes particularly at zones of valley confinement.

Hydrogeomorphic controls of early post-logging successional pathways in a southern floodplain forest

Successful management or restoration of southern floodplain forest ecosystems requires an understanding of how subtle changes in environmental conditions such as site hydroperiod affect species suitability as well as the importance of recruitment limitation. In this research, we examined the effects of elevation, modeled flood regime, soil conditions and forest proximity on early, post-logging succession in clearcuts on the Congaree River floodplain near Columbia, South Carolina. Analyses of community composition clearly demonstrated that regeneration was structured along environmental gradients associated with site hydroperiod, with obligate wetland species dominating sites with the lowest elevations and deepest projected flood inundation depths. The effects of soil characteristics on site composition were in some cases inter-correlated with site hydroperiod, reflecting the influence of flood regime on soil drainage, aeration, and soil redox potential. In other cases, though, site composition was distinguished along a second, but equally important, orthogonal axis that was more reflective of the effects of geomorphic history on edaphic conditions. There was no evidence that fine-scale dispersal limitation affected succession, in part because of the effects of resprouting, but it is possible that past human activities have altered landscape-scale patterns of commercially valuable species such that seed and sprout sources for advance regeneration may have been absent at the time of the most recent clearcutting. Our examination of regeneration patterns, processes and controls clarified the relationships between current conditions and community composition, but, because patterns of elevation, flooding and soil characteristics across a floodplain have their roots in geomorphic processes, they are also ultimately a reflection of past hydrogeomorphic events.

Empirical modeling of light availability in rivers

While the influence of hydrology and geomorphology on ecosystem‐limiting factors in rivers has been well studied, particularly habitat availability and nutrient cycling, the more fundamental limitation of light availability has received much less attention. Characterizing light regimes in rivers is optically complex and requires consideration of five hydrogeomorphic controls: topography, riparian vegetation, channel geometry, optical water quality, and hydrologic regime. To generalize and quantify these hydrogeomorphic controls, we developed an empirical model that predicts both spatial and temporal variability of photosynthetically active radiation reaching the riverbed (benthic PAR). We applied this benthic light availability model (BLAM) to two dissimilar systems: a large, turbid river and a small, optically clear river. Comparisons between the two systems revealed that the dominant control on temporal light availability for the large river was discharge, which accounted for 90% of the variation. A dominant temporal control for the small river did not emerge, but instead was found to be a function of both above‐canopy PAR and discharge. Spatially, water depth accounted for 99% of the variation in benthic PAR for the large river, and riparian shading accounted for 93% of the variation for the small river. Channel orientation also had a major influence, where an E‐W configuration increased benthic PAR by as much as 108% relative to a N‐S configuration. BLAM predictions agreed well with measured benthic PAR, within 39% on average over a 9‐d period. BLAM is the first model to quantify benthic PAR using all five hydrogeomorphic controls, and thus provides a new tool for investigating the role of light in fluvial ecosystem dynamics and for establishing light availability targets in water resource management.

Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: A review of complementary approaches

Until recently, one-way relationships between flow dynamics, geomorphology and plant ecology were considered dominantly when studying the functioning of river systems, whereby fluvial landforms and hydrogeomorphic processes drive the evolution of riparian plant communities. However, biological communities may significantly control geomorphic processes and have strong impacts on landform dynamics. In order to fully identify the processes linked to river dynamics (changes in time and space of fluvial landforms and associated plant communities), conceptual multidisciplinary progress is clearly needed. To understand the mutual interactions and feedbacks between fluvial landforms and vegetation community dynamics, this paper presents a detailed literature review of fluvial geomorphology, riparian plant ecology and hydraulic engineering knowledge. The historical and recent development of ecological plant succession theory toward the integration of hydrogeomorphic disturbances is discussed as well as the integration of vegetation within geomorphology as a significant landform control factor, incorporating both hydrogeomorphic controls on riparian vegetation dynamics and mechanical impacts of vegetation structures on flow properties and sediment dynamics. Recent progress in ecology, hydraulic engineering and fluvial geomorphology emphasises interdependence between biological and physical forms and processes. Based on this literature review, a ‘fluvial biogeomorphic succession’ concept is proposed to link fluvial landform and riparian vegetation community evolution within a bi-directional model. The succession of fluvial landforms and associated vegetation communities is composed of four main critical phases that represent a shift in the relative dominance of hydrogeomorphic and ecological processes as a response to biostabilisation and passive bioconstruction processes. The positive feedbacks associated with this shift lead to the development of characteristic biogeomorphic structures such as vegetated banks, islands or floodplains, which are moderated by the biogeomorphic functional roles of ‘ecosystem engineers’ that induce or reinforce the positive feedbacks. This fluvial biogeomorphic succession concept relates the natural Darwinian selection and ecological succession theories to fluvial geomorphology.

Hydrogeomorphic Controls on Reduction–Oxidation Conditions across Boreal Upland–Peatland Interfaces

The reduction–oxidation (redox) state of peatland pore waters plays an important role in many peatland biogeochemical processes. Recent research has also shown that the interface between ecosystems, or the ecotone, may be responsible for a disproportionate amount of biogeochemical activity when material and/or energy is hydrologically transported between ecosystems. The purpose of this research was to examine the spatiotemporal dynamics of redox conditions across two geomorphically distinct Boreal Precambrian Shield upland–peatland ecotones to determine the spatial and temporal scales at which these ecotones may be important. Pore water chemistry of iron and sulphur species was monitored across two upland–peatland ecotones in northwestern Ontario in conjunction with hydrological measurements under both stormflow and nonstormflow conditions. In addition, one upland–peatland ecotone was instrumented to make continuous measurements of in situ redox potential (Eh) over a 12-day period to determine whether measurements at a high temporal scale could provide additional insights into the transfer of nutrients across the upland–peatland interface. Results indicated that hydrology—specifically, groundwater flowpath and the strength of the hydrological connection between upland and peatland—determined the spatial extent of the ecotone as a biogeochemical hotspot. In situ Eh measurements showed that these ecotones may be most important over a scale of only several hours and are largely affected by lateral hydrological flows from the upland. The role of both hot spots and hot moments in biogeochemistry must be considered to accurately estimate the ability of a single ecosystem to process chemical inputs.

Hydrogeomorphic controls on phosphorus retention in streams

We compared the relative influences of biochemical uptake processes and dynamic hydrology and geomorphology (hydrogeomorphology) on molybdate reactive phosphorus (MRP) retention within a stream. MRP concentrations were measured upstream and downstream of a 4.5‐km reach undergoing dynamic channel adjustment in response to downstream dam removal. Geomorphic adjustments following removal produced measurable changes in velocity and depth, and decreases in MRP retention. Paired upstream and downstream measurements of MRP concentration were used to compute three retention metrics: uptake rate, mass transfer coefficient, and uptake length, which were used as model parameters. Modeling results showed that changes in channel morphology alone following dam removal could result in an approximate 40% increase in downstream MRP concentrations compared with conditions with the dam in place. However, empirical and modeling results indicate that hydrogeomorphology can control nutrient retention on the reach scale only when uptake processes are either sufficiently great or when uptake rates have limited variability. Review of published phosphorus retention values revealed greater variability in biochemical uptake rates than in hydrogeomorphology. Thus uptake rates should exert a stronger control on reach‐scale MRP retention than changing channel morphology or hydrology. These results suggest that maintaining or restoring channel conditions that are conducive to biochemical uptake are of greater priority than restoration of hydrologic or geomorphic conditions alone.

Piping susceptibility and the role of hydro-geomorphic controls in pipe development in alluvial sediments, Central Spain

Piping susceptibility and the role of hydro-geomorphic controls in pipe development in alluvial sediments, Central Spain

Morphometry and floods in small drainage basins subject to diverse hydrogeomorphic controls

Morphometric parameters such as drainage density, stream magnitude, and relief ratio are practical measures of flood potential in small (&lt;100 mi2) drainage basins. Stereoscopic interpretation of low‐altitude aerial photographs provides the most accurate maps of basins for generating these parameters. Field surveys of a high‐density limestone basin in central Texas show that 1:24,000 scale topographic maps accurately portray the efficient stream channel system but fail to reveal numerous small gullies that may form portions of hillslope hydrologic systems. Flood potential in drainage basins can be defined by a regional index computed as the standard deviations of the logarithms of the annual maximum streamflows. High potential basins tend toward greater relief, greater drainage density, and thus greater ruggedness numbers than low–flash flood potential watersheds. For a given number of first‐order channels (basin magnitude), flash flood regions have greater ruggedness numbers, indicating higher drainage densities combined with steep hillslopes and stream channel gradients. Transient controls on flood response, such as differences between local rainstorm intensities, appear to be the major influences on hydrographs in areas of moderate dissection and relief. Morphometric parameters for low‐potential flash flood regions )Indiana and the Appalachian Plateau( are better estimators of frequent low‐magnitude runoff events (mean annual flood), while the same parameters correlate better with the maximum flood of record in high‐flood potential regions (central Texas, southern California, and north central Utah).