Riparian Vegetation Communities Research Articles

Does dam construction increase the biotic homogenization of riparian vegetation along a regulated river?

AbstractDam construction in natural rivers alters the intensity and frequency of water level fluctuations in reservoirs and their downstream river reaches, leading to the biotic homogenization of riparian communities. Although the intensity of dam‐induced water level fluctuations is often described qualitatively, quantitative analysis of its effects on riparian vegetation remains inadequate. This study compared riparian species dissimilarity between the natural Chishui River (as a control) and the regulated Wujiang River (with cascade reservoirs) to assess the level of biotic homogenization under different regulation types and in zones with different riverbank characteristics using the Bray–Curtis dissimilarity index. The reservoirs along the Wujiang River were classified into four different regulation types according to the water retention time, which were daily (0–7 days), weekly (7–30 days), seasonally (30–90 days), and yearly (>365 days). Results showed significant species dissimilarity in riparian vegetation along the Wujiang River compared to that along the Chishui River in the inundation, transitional, and unflooded zones. Furthermore, the species composition and dissimilarity patterns differed in the reservoirs and downstream rivers of the cascade dams in terms of regulation type and zone. For different regulation types, there was high species dissimilarity in both the upstream and downstream inundation zone of the daily reservoirs, while species dissimilarity was lower in both upstream and downstream of the yearly reservoir (Bray–Curtis index <0.5). This study provides a quantitative measure of how dam construction perturbs riparian vegetation communities, emphasizing the importance of the balance between the riparian ecosystem conservation and the economic demands of dam construction.

Environmental Flow Increases The Riparian Vegetation Diversity And Community Similarity

Environmental Flow Increases The Riparian Vegetation Diversity And Community Similarity

The Effects of Legacy Sediment Removal and Floodplain Reconnection on Riparian Plant Communities

Stream restoration includes a number of different approaches intended to reduce sediment and nutrient export. Legacy sediment removal (LSR) and floodplain reconnection (FR) involve removing anthropogenically derived sediment accumulated in valley bottoms to reconnect incised streams to their floodplains. These projects also present an opportunity to create high-quality riparian and wetland plant communities and provide information about the early stages of wetland vegetation development and succession. We surveyed vegetation immediately after restoration at three sites and at three additional sites 1–3 years post-restoration to determine how LSR/FR affects riparian plant communities. Restoration increased the prevalence of hydrophytic herbaceous species at all sites, suggesting these projects successfully reconnected the stream to the floodplain. Pronounced decreases in woody basal area and stem density likely also influenced an increase in native and graminoid species after restoration. Only 16% of the indicator species identified for restored reaches were planted as part of the restoration, suggesting the local seed bank and other seed sources may be important for vegetation recovery and preservation of regional beta diversity. Although vegetation quality increased after restoration in reaches with initially low-quality herbaceous vegetation, vegetation quality did not improve or decreased after restoration in reaches with higher-quality vegetation before restoration. The practice of LSR/FR has the potential to improve the quality of some riparian vegetation communities, but the preservation of high-quality forested areas, even if they are atop legacy sediment terraces, should be considered, particularly if reductions in nutrient export do not offset losses in tree canopy.

Landscape regulation of microbial use of terrestrial carbon in boreal streams

Microbes decomposing leaf litter in aquatic ecosystems are exposed to two major sources of carbon (C), namely, particulate organic C (POC) and dissolved organic C (DOC). The use of DOC relative to POC during litter decomposition likely depends on the availability of DOC, which in turn is regulated by the characteristics of the surrounding landscape, although this extrinsic indirect control of DOC use remains largely unexplored. We have investigated how variations in stream physical and chemical characteristics, distribution of major landscape elements (i.e., forest, mires, and lakes), and riparian vegetation community composition (i.e., relative cover of deciduous vs. coniferous tree species) influence DOC use by leaf-associated microbes (LAM). Specifically, in a boreal stream network of ten first- to third-order streams, we related in-stream characteristics, landscape elements, and riparian vegetation community composition to DOC/POC respiration ratios (i.e., the amount of CO2 produced by LAM respiration of DOC + POC divided by the amount of leaf C mass lost through decomposition). The results showed that DOC/POC respiration ratios were > 1 in most of the study sites, indicating that LAM use a substantial amount of DOC during leaf litter decomposition. This microbial reliance on DOC varied with in-stream DOC and nutrient concentrations, proportional mire and forest cover, and riparian vegetation community composition. In particular, high mire and coniferous cover increased DOC use by LAM. As such, landscape configuration and how it is modified by land use and climate change must be considered in order to understand microbial turnover of terrestrial C in boreal streams.

Evaluating Riparian Plant Communities After Restoration of Plains Bison in the Northern Great Plains of Montana

Restoration of plains bison (Bison bison bison) in the northern Great Plains has been controversial for a variety of reasons, including the public concern that bison will supplant cattle on public rangelands. Riparian zones are among the most biologically rich areas within rangelands, but they are highly sensitive to disturbances such as grazing, leading to the public perception that riparian vegetation communities will be negatively impacted by year-round bison grazing compared with the norm of seasonal cattle grazing. Our objective was to evaluate the vegetation community and soil compaction conditions of areas in the northern Great Plains of Montana, where bison have been restored 5−10 yr as year-round grazers, compared with adjacent sites where cattle have been retained as seasonal grazers. Out of the 24 variables we assessed, only 2 significantly differed between the grazer treatments within our study's time frame and stocking rates. Native species diversity was significantly higher in the bison-grazed treatment (F[1,1,5.98] = 8.16, P = 0.03), and woody height heterogeneity was twice as high in the bison-grazed treatment than the cattle-grazed treatment (F[1,5.97]= 5.81, P = 0.05). Although longer-term studies are required, our findings indicate that year-round bison grazing under the current stocking rates did not degrade riparian vegetation communities compared with seasonal cattle grazing and rather benefitted some aspects of the plant community.

Riparian vegetation response amid variable climate conditions across the Upper Gila River watershed: informing Tribal restoration priorities

Riparian systems across the Southwest United States are extremely valuable for the human and ecological communities that engage with them. However, they have experienced substantial changes and stresses over the past century, including non-native vegetation expansion, vegetation die-offs, and increased fire activity. Vegetation management approaches, such as ecological restoration, may address some of these stressors as well as reduce the risk of future impacts. We apply remote sensing to inform restoration priorities along the upper Gila River within the San Carlos Apache Reservation and Upper Gila River watershed. First, we develop a spatially and temporally explicit trend analysis across three observed climate periods (1985–1993, 1993–2014, 2014–2021) using the Landsat-derived indices to quantify changes in riparian vegetation conditions. These maps can be used to identify areas potentially more at risk for degradation. Second, we analyze changes in riparian vegetation within a climate framework to better understand trends and the potential effect of climate change. Vegetation greenness has largely increased throughout the watershed despite intensifying drought conditions across our study period, though areas within the lower watershed have shown increased stress and higher rates of wildfire and other disturbances over the past 5-years. Nevertheless, small-scale restoration activities appear to show improving vegetation conditions, suggesting efficacy of these restoration activities. Results from this study may be integrated with restoration objectives to develop a restoration plan that will help riparian vegetation communities adapt to change.

Quality of riparian habitat and its influence on water quality in some ecosystems of waterfalls and springs in Bawean Island

This study aims to evaluate the quality of riparian habitats based on the index of riparian quality (QBR), quality of riparian tree vegetation (community structure and Naturalness index), and anthropogenic activity (Hemeroby Index) and their relationship to water quality in some waterfall and spring ecosystems on Bawean Island. This Ex Post Facto study was conducted on six waterfall ecosystems (Candi, Putri, Suwari, Talomon, Laccar and Kuduk Kuduk) and two spring water ecosystems (Kuduk Kuduk and Laccar). Analysis of riparian tree vegetation at each location was observed from three plots with size 50 x 50 m2. The physics chemical quality of water observed at each location included pH, dissolved oxygen/DO, total suspended solid/TSS, total Kjeldahl nitrogen/TKN, and biochemical oxygen demand/BOD. The results showed that the quality of the riparian habitat affected the water quality.

Spatial patterns of riparian vegetation community composition and diversity along human‐ affected East African highland streams

AbstractRiparian vegetation, which plays important roles in conservation of regional biodiversity and provision of many environmental services, has been severely degraded in East Africa by human activities. To ameliorate this degradation, more knowledge of the vegetation and factors affecting it is required. Thus, effect of land use on the plant community composition, species richness and diversity patterns were investigated along 18 streams in the Gilgle Gibe River catchment, in south‐western Ethiopia, using 100 m2 plots established along transects on both sides of the streams at 35 sampling locations beside land designated as agricultural, forested, mixed vegetation or eucalyptus plantation. The communities in the plots were surveyed and classified by Two‐Way Indicator Species Analysis. In total, 107 vascular plant species belonging to 49 families were recorded. Species richness and diversity were lowest along streams beside agricultural land, which had narrow riparian buffers, and highest along forested streams, which had wider riparian buffers. The communities in the sampling plots were assigned to seven groups. Species richness was positively correlated buffer width (r = 0.74, p < 0.01). The results highlight the human influence on riparian vegetation and the importance of sustainable management that is compatible with its conservation and restoration. However, to address the severity and complexity of forest fragmentation, conservation strategies must embrace a multi‐site, contextual approach.

Density of Axis Deer in Texas: Management Implications for Native White-tailed Deer and Associated Habitats

Abstract Axis deer Axis axis have been widely introduced to new geographic ranges, and in the United States, free-ranging axis deer have become well established in the Edwards Plateau ecoregion as well as other portions of Texas. However, no estimates of axis deer population density nor size have been conducted since 1994. It is hypothesized that axis deer on the Edwards Plateau are potentially competing with native white-tailed deer Odocoileus virginianus for food, space, and habitat resources, and causing damage to important riparian habitats. Our goal was to estimate regional densities of axis deer and white-tailed deer, and provide insight about the potential impacts axis deer may have on native wildlife and their habitats. Estimated using distance sampling techniques in 2018 and 2019, average axis deer density was 19.7 (95% CI: 14.1–25.6) axis deer/km2 compared with 23.0 (95% CI: 18.2–27.5) white-tailed deer/km2, and axis deer densities ranged from 16.9 to 171.0/km2 among eight different land cover types in Kimble County, Texas, with a county-wide estimate of 61,078 (95% CI: 30,407–100,369) axis deer. Axis deer densities were greatest in riparian land cover types, and they selected for two riparian land cover types and upland grasslands. Axis deer population estimates clearly indicate their population size has increased substantially since introduction to Texas in the 1930s. Population management of axis deer is warranted to limit impacts to native wildlife from potential habitat usurpation, or damage to riparian vegetation communities, soil, and water quality.

Wetland vegetation regeneration in response to tidal reinstatement on an abandoned sugar cane farm

Supratidal wetlands are threatened by agricultural production and are highly vulnerable to climate change, particularly through sea level rise (SLR). While vegetation structure and composition of supratidal wetlands will likely change under projected SLR with run‐on effects for ecosystem service provision, these changes can provide opportunities for restoration of adjacent agricultural land. Here, we investigated the natural regenerative potential of supratidal wetlands on abandoned agricultural land in Southeast Queensland, Australia, specifically, responses of wetland vegetation communities to simulated SLR, through tidal reinstatement. In 15 years since crop abandonment, distinct communities of typical supratidal wetland vegetation have naturally reestablished, in predominately freshwater conditions, with minimal management intervention. Reinstating tidal floodwater increased the flooded extent and permanence of brackish water. Four repeat surveys of vegetation composition, structure, and condition were conducted in permanent plots established in Casuarina swamp, Melaleuca swamp, herbaceous marsh, and riparian zone vegetation communities, to observe change over time. Species richness decreased in all regenerating communities (Herbaceous marsh, Casuarina, and Melaleuca) post flood gate removal. Understory vegetation cover also decreased in Melaleuca and Casuarina plots, but increased in herbaceous marsh plots, with increased cover of salt‐tolerant species throughout. Changes in woody vegetation community and structure were not observed during this short study (2.5 years), although the regenerative capacity of woody and herbaceous species was reduced. Supratidal wetland vegetation communities can naturally reestablish in areas of abandoned agricultural land, however, increased saltwater flooding (likely with SLR) will put these communities at risk of transition to salt‐tolerant vegetation.

The hydroclimate niche: A tool for predicting and managing riparian plant community responses to streamflow seasonality

AbstractHabitat suitability is a consequence of interacting environmental factors. In riparian ecosystems, suitable plant habitat is influenced by interactions between stream hydrology and climate, hereafter referred to as “hydroclimate”. We tested the hypothesis that hydroclimate variables would improve the fit of ecological niche models for a suite of riparian species using occurrence data from the western United States. We focus on the climate conditions (temperature, precipitation and vapor pressure deficit) during the months of lowest and highest streamflow as integrative hydroclimate metrics of resource and stress levels. We found that the inclusion of hydroclimate variables improved model fit for all species in the western USA dataset. We then tested the utility of the improved habitat suitability models by projecting them onto a regulated segment of the Colorado River to assess potential impacts of streamflow seasonality on vegetation metrics of management concern. Species frequency derived from independent survey data in the Colorado River segment was significantly higher for species with predicted suitable habitat than for species without predicted suitable habitat. Under different simulated hydrographs for the Colorado River, overall species richness was predicted to be greatest with peak streamflows during summer, and native‐to‐non‐native species ratios were predicted to be greatest with lowest streamflows in winter. Summer high flows were particularly associated with higher predicted habitat suitability for species that have increased in cover over recent decades (e.g., Pluchea sericea, Baccharis species). We conclude that hydroclimate covariates can be useful tools for predicting how riparian vegetation communities respond to changes in the seasonal timing of low and high streamflows.

Building resilient landscapes in a semi-arid watershed: Anthropogenic and natural burning histories in Late Holocene Tesuque Creek, northern New Mexico

As mega-fires have swept the North American West in recent decades, studies of past fire events have gained academic interest. Deep-time perspectives are necessary to better understand the periodicity of fire events and to identify basic drivers of frequent fire episodes through time, including potential anthropogenic fire use. Our study contributes to the growing field of archeological fire ecology in the Southwest with an examination of a high-resolution geobotanical dataset for Holocene fire regimes as indicated within sediments from an alluvial section from Tesuque Creek in northern New Mexico. Using phytolith, geochemical, and sedimentological data, we reconstruct indices for burn cycle seasonality and destabilization of riparian vegetation communities that suggest potential anthropogenic land use and possible patterns distinguishing human burning from lightning ignitions. Through this study, we consider the potential role of hunter-gatherer behavior in producing fire landscapes as part of their strategies for managing risks in the fragile environments of the Middle-Late Holocene in northern New Mexico. We suggest that combining geo and botanical datasets from alluvial sections with archeological evidence will allow researchers to fine-tune the resolution of local climate sequences and their environmental impacts on small landscape scales. When such small-scale studies can be aggregated, they may further contribute to testing models of regional patch-burning, either natural or anthropogenic. We further posit that human relationships with fire in the past should be considered as collaborative efforts with lightning-ignited burns, to manage the risks associated with both too much and too little fire in fragile dryland ecosystems.

A tale of two rivers: Dam‐induced hydrologic drought on the lower Dolores River and its impact on tamarisk establishment

AbstractThe effects of river regulation on the flow patterns of rivers in western North America often mimic the impacts of naturally occurring episodic drought. On the Dolores River in southwestern Colorado, two distinct periods of flow modification have occurred, with very different drought‐like consequences for downstream habitat. The first period of major irrigation diversions (1889–1984) led to extreme low flows during the summer months with little change to spring peak flows. The second period after dam construction (1984 to present) caused a decline in peak flows but an increase in summer minimum flows. We studied the role of these different flow regimes in promoting the recruitment of non‐native tamarisk, since understanding historic patterns of recruitment may inform current and future flow management. We used dendrochronology to determine tamarisk establishment dates and tested the connection between establishment years and a suite of hydroclimatic variables. Tamarisk recruitment occurred between 1952 and 2002, with 92% of trees establishing during the first flow diversion period. The strongest hydroclimate predictor was low minimum flow for a given year, especially when coupled with higher peak flows in the year of, or before, establishment. Almost no tamarisk recruitment has occurred in the second, dam‐controlled period, where lowered peak flows and elevated minimum flows prevail. In spite of the drought tolerance of tamarisk, drought‐like conditions below this dam did not promote its invasion. Instead, the combination of lowered peaks but increased summer baseflows post‐dam has driven channel narrowing and vegetation encroachment by native willows. What current and future meteorologic drought conditions, coupled with dam‐controlled flows, will bring to the channel and adjacent riparian vegetation is uncertain, but this work may help to forecast the impacts of future river flows on riparian vegetation community evolution.

Biodiversity in Urban Areas: The Extraordinary Case of Appia Antica Regional Park (Rome, Italy).

The first inventory of the flora of Appia Antica Regional Park (Italy), one of the largest protected urban areas in Europe (4580 ha), its biological, ecological and biogeographical composition, and notes of the vegetation physiognomies and landscape are presented; physical characteristics of the territory (geomorphology, lithotypes, and phytoclimate) are also given. The landscape is defined by an agricultural matrix with natural and seminatural areas as patches, and riparian vegetation communities as corridors. The vegetation physiognomies are represented by types linked to the Mediterranean climate (mixed, Mediterranean, and riparian forests; scrubby, rocky, aquatic, and helophytic vegetation; anthropogenic communities). The floristic list includes 714 taxa (104 families and 403 genera). Therophytes prevail over hemicryptophytes; woody flora comprises about 30% of alien species. As regards chorotypes, together with a considerable number of Mediterranean species, there are many exotic species with wide distribution areas testifying to a long-lasting anthropic impact. Floristic novelties (european, national, and regional levels) for 21 taxa are reported. The extraordinary species diversity discovered (43% of flora of Rome and 20% of regional flora) is linked to the landscape heterogeneity, the characteristics of which are: (1) persistence of residual natural patches, (2) occurrence of quite well-preserved aquatic habitats and humid meadows, (3) a rich anthropogenic flora, (4) an interesting flora of archeological sites, (5) occurrence of species not common in Latium, (6) occurrence of populations of aliens in crops (which cause economic impact), (7) presence of aliens on archeological ruins (which cause economic-social impacts). The extensive set of data provided represents a general base framework for guiding future research efforts and landscape action plans consistent with environmental sustainability.

Measurement of foliar H2O2 concentration can be an indicator of riparian vegetation management

Riparian vegetation is frequently exposed to abiotic stress, which generates reactive oxygen species (ROS) caused by strong differences in a river’s hydrological conditions. Among different ROS, hydrogen peroxide (H2O2) is relatively steady and can be measured appropriately. Thus, the quantification of plant H2O2 can be used as a stress indicator for riparian vegetation management. The current study examines the spatial distribution of plants by riparian vegetation communities across the elevation gradient of riparian zones through quantification of environmental stress using foliar H2O2 concentration. The trees Salix spp., Robinia pseudoacacia, Ailanthus altissima with Juglans mandshurica, and the herbs Phragmites australis, Phragmites japonica, and Miscanthus sacchariflorus were selected for this study. Leaf tissues were collected to analyze H2O2 concentration, meanwhile riparian soil was sampled to measure total nitrogen (TN), total phosphorus (TP), and moisture content. The H2O2 concentration of tree species increased with higher soil moisture content, which was negatively correlated for Salix and herb spp., in which H2O2 concentration always decreased with high soil moisture. In this study, we found a unique significant interaction between soil moisture content and H2O2 concentration, both positively or negatively correlated relationships, when compared with other parameters, such as TN or TP concentrations or TN: TP in riparian soil. The species-specific distribution zones can be explained by the H2O2 concentration in the plant for gravelly and sandy channels on a theoretical range of soil moisture. Each species’ H2O2 concentration was estimated through derived equations and is directly related to an elevation above the channel. The comparison with the observed distribution of plant elevations in the field indicated that all species showed a spatial distribution that acts as species-specific elevations where H2O2 concentrations stayed below 40 μmol/gFW. Hence, the present study suggests that foliar H2O2 concentration can be a useful benchmark for the distribution potentiality of riparian vegetation.

Ultrashort-term responses of riparian vegetation restoration to adjacent cycles of ecological water conveyance scheduling in a hyperarid endorheic river basin

The management of ecological water conveyance (EWC) can allow riparian vegetation communities to survive the threat of degradation in hyperarid inland areas and promote the health of groundwater-recharged riparian ecosystems. However, the ultrashort-term effects of periodic EWC scheduling on riparian vegetation remain unclear. This study explored the spatiotemporal differentiation in species structure (herbs, shrubs, and trees), diversity (measured by the Simpson, Shannon–Wiener, Pielou, and Margalef indices), stability (evaluated via Godron fitting distances and abundance-biomass comparison curves), and integrity (proxied by the vegetation-based index of biotic integrity) of vegetation communities in the downstream Heihe River Basin, China. Empirical orthogonal function, Pearson correlation, canonical correspondence analysis (CCA), and partial CCA methods were used to evaluate the effects of dominant habitat environmental factors from the hydrogeographic features, soil physicochemical properties, and anthropogenic impacts. The results showed that the riparian vegetation community diversity, stability, and integrity varied moderately to slightly with hierarchical distance from near wetlands (<200 m; containing mainly herbs) to far desert edges (>800 m; occupied by shrubs/subshrubs). The middle transition zone (200–800 m; occupied mostly by trees/subtrees) had the best diversity and integrity but relatively poor stability. The most significant influencing factors were EWC and soil moisture. The simple diversity, fair-level integrity, and disturbed but not irreversibly damaged stability of the vegetation community were generally improved by 14.82%, 20.33%, and 30.57%, respectively, in the pre-EWC period but worsened in the post-EWC period. The difference in spatially distributed EWC quantities caused more apparent vegetation restoration in high water-supplied subareas where certain biological community instability existed. Therefore, adequate EWC management can be considered a prerequisite for the maintenance of high richness and structural stability in local communities and requires a good balance between interregional vegetation abundance and enhanced environmental tolerance.

A Green New Balance: Interactions among riparian vegetation plant traits and morphodynamics in alluvial rivers

AbstractThe strength of interactions between plants and river processes is mediated by plant traits and fluvial conditions, including above‐ground biomass, stem density and flexibility, channel and bed‐material properties, and flow and sediment regimes. In many rivers, concurrent changes in (1) the composition of riparian vegetation communities as a result of exotic species invasion and (2) shifts in hydrology have altered physical and ecological conditions in a manner that has been mediated by feedbacks between vegetation and morphodynamic processes. We review how Tamarix, which has invaded many southwestern US waterways, and Populus species, woody pioneer trees that are native to the region, differentially affect hydraulics, sediment transport, and river morphology. We draw on flume, field, and modelling approaches spanning the individual seedling to river‐corridor scales. In a flume study, we found that differences in the crown morphology, stem density, and flexibility of Tamarix compared to Populus influenced near‐bed flow velocities in a manner that favoured aggradation associated with Tamarix. Similarly, at the patch and corridor scales, observations confirmed increased aggradation with increased vegetation density. Furthermore, long‐term channel adjustments were different for Tamarix‐ versus Populus‐dominated reaches, with faster and greater geomorphic adjustments for Tamarix. Collectively, our studies show how plant‐trait differences between Tamarix and Populus, from individual seedlings to larger spatial and temporal scales, influence the co‐adjustment of rivers and riparian plant communities. These findings provide a basis for predicting changes in alluvial riverine systems which we conceptualize as a Green New Balance model that considers how channels may adjust to changes in plant traits and community structure, in addition to alterations in flow and sediment supply. We offer suggestions regarding how the Green New Balance can be used in management and invasive species management.

Beta Diversity Patterns Unlock the Community Assembly of Woody Plant Communities in the Riparian Zone

Beta diversity refers to changes in community composition across time and space, including species richness and replacement. Few studies have examined beta diversity patterns of riparian vegetation communities in terms of taxonomic, phylogenetic and functional attributes. In this study, we conducted a field survey of woody plant communities in the riparian zone of the Lijiang River Basin in China. We analyze variations in taxonomic, phylogenetic and functional beta diversity, the relative contributions of species richness and replacement to beta diversity and the relationships between beta diversity and environmental distance and geographical distance. The results show that: (1) replacement was the dominant component of taxonomic beta diversity and richness was the dominant component of functional and phylogenetic beta diversity; and (2) dispersal limitation and habitat filtering jointly drive the community assembly of woody plant communities in the riparian zone of the Lijiang River Basin. Therefore, when formulating conservation strategies for woody plants along the Lijiang River riparian zone, improving ecological communities and enhancing species dispersal between communities should be given equal attention. From a taxonomic perspective, it is more suitable to establish several small nature reserves, whereas from phylogenetic and functional perspectives, protection should focus on larger nature reserves.

Urban runoff and stream channel incision interact to influence riparian soils and understory vegetation.

Riparian soil processes and vegetation are sensitive to water availability. Urbanization can alter riparian water availability by modifying stream flows and stream channel morphology. In cities, runoff from impervious surfaces tends to increase stormflow magnitudes, causing stream channels to incise, or downcut. This change in channel morphology has been linked to lowered water tables and drier conditions in temperate urban riparian zones, leading to shifts in riparian nitrogen (N) cycling and vegetation communities. In Mediterranean climates with distinct wet and dry periods, there is an additional dynamic to consider: runoff from urban water use can cause streams to flow when they would otherwise be dry. This dry-season stream flow could create increased, rather than decreased, water availability in urban riparian zones. However, channel incision may counteract this effect. We asked whether dry-season stream flow interacted with channel incision to influence riparian soil characteristics and understory vegetation along streams in Sacramento, California, which has a Mediterranean climate with an intense summer dry season. At 40 stream reaches that varied by severity of downcutting and presence of dry-season flow, we sampled soils and vegetation on top of stream banks and at the margin of the low-flow channel, an important location for nutrient cycling in dry climates. We measured soil moisture, organic matter, and ∂15 N, as well as total and perennial understory vegetation cover. We found that channel characteristics associated with incision limited the influence of dry-season stream flow on soil moisture, and this interaction appears to have lasting effects on soil organic matter and perennial vegetation on bank tops. At the stream margin, channel downcutting was associated with reduced soil organic matter and vegetation cover, while dry-season flow was associated with increased vegetation cover. Values of soil ∂15 N pointed to limited hydrologic linkage between stream flows and riparian bank soils along incised streams. Our findings suggest that channel incision could limit the ability of urban riparian ecosystems to mitigate low-flow water quality. However, where streams are not incised in Mediterranean climates, dry-season flows from urban runoff may actually increase riparian productivity and N cycling above historical levels.

Hydrogeomorphic Scaling and Ecohydraulics for Designing Rescaled Channel and Floodplain Geometry in Regulated Gravel–Cobble Bed Rivers for Pacific Salmon Habitat

Societies are increasingly restoring and/or rehabilitating rivers below dams for keystone species such as salmon. A fundamental concept for rehabilitating river morphology below dams for salmon is that a rescaled version of the river corridor synchronized to the regulated flow regime can restore habitat quantity and quality. Downscaled and resized hydrographs have been shown to provide environmental benefits to fish communities including salmon as well as riparian vegetation communities. However, less research exists on how this can be achieved through the topographic rescaling of heavily modified and regulated river corridors. The goal of this paper is to review analytical methods to determine initial of size of rescaled channel and floodplain mesohabitat units in regulated gravel–cobble bed rivers for Pacific salmon (Oncorhynchus spp.) habitat using hydrogeomorphic scaling and ecohydraulics. Hydrogeomorphic flow scaling is the prediction of river morphology and geometry using empirical and analytical relationships. Ecohydraulic scaling refers to the use of ecohydrology, habitat suitability curves, and fish density relationships to determine the size of mesohabitat units for ecologically relevant flows. In practice, these are complimentary first order estimates of channel and floodplain configurations followed by iterative design in a hierarchical manner. This review advances the science of river design by synthesizing these complimentary ideologies for Pacific salmon habitat restoration in regulated rivers. Following the review, the layout of features is briefly discussed followed by a discussion of important considerations beyond the physical and topographic rescaling of river corridors for salmonid habitat restoration.