Unvegetated Bars Research Articles

Assessing functional characteristics of a braided river in the Qinghai-Tibet Plateau, China

Functional characteristics of braided rivers with very high elevations (>3400 m) located in the Qinghai-Tibet Plateau, China provide unique insight into fully understanding global braided river functioning, but have not yet been explored because of their remote locations and harsh physiographic conditions. In this study, we unveiled these characteristics at the reach spatial scale over three decades in one of these braided rivers, the Upper Lancang River (ULR). Using Google Earth Engine resources and a specific selection of Landsat images obtained between 1989 and 2018 during the dry season (November to early May when water levels were low and comparable over years), we analyzed water area, extracted flowing and non-flowing channels, active channel widths (unvegetated bars and water channels), and calculated lateral shifting rates of the main channel for 13 sub-periods. We also developed an empirical method for estimating vegetated areas in the selected years. We found that (1) this braided system is partly controlled by peak flows and flood durations and the braiding activity evolved through time according to braided planform characteristics, (2) braiding intensity is high during low discharges, suggesting that the ULR is a very well-connected braided system with groundwater, but we observed higher braiding activity in spring than in fall, suggesting other factors than groundwater recharge also control non-flowing channel occurrence, and (3) the ULR supports a vegetation mosaic that is dynamic at a multi-year scale with period of declines that can be caused by scouring and rapid growth in an optimal recruitment window. These findings lead to our interpretation of vegetation behavior and surficial water – groundwater interactions, which needs future field validation.

Can we characterize river corridor evolution at a continental scale from historical topographic maps? A first assessment from the comparison of four countries

AbstractNational historical map resources are assessed in four European countries to characterize river corridor features and associated channel changes, as well as identify issues limiting or promoting geomorphic assessment procedures at a continental scale. A geomorphic audit that launches potential data for diagnosis from reach to continental scales could offer a good resource for biology and ecology managers of river authorities or government agencies and engineers. The assessment compares the resources available by country in terms of period covered, spatial scale, history and chronology, and representation of the fluvial corridor features. We then applied the Historical Maps Vectorization Toolbox, initially developed for vectorizing river corridors from French maps, to detect and extract flow channels, unvegetated bars and riparian vegetation patches from historical topographical maps. We found that (a) it is difficult to apply an audit of channel changes to the whole continental scale because map legends differ between countries due to geographic and political specificity; (b) there exists an opportunity to get assessment information in all countries at reach or national scale where map resources are available; (c) the highest potential is observed in Switzerland and Belgium where there is high quality national map coverage from the 19th century; and (d) the algorithm Historical Maps Vectorization Toolbox applied to map resources works well with any of the countries, and its widespread application is encouraging.

Mountain river restoration measures and their success(ion): Effects on river morphology, local species pool, and functional composition of three organism groups

We investigated temporal effects of restoration on river morphology, on species and functional composition of benthic invertebrates, floodplain vegetation and carabid beetles at three study sites in the mountain river Lahn (Germany). We sampled restored and nearby non-restored sections 3–5 years and 7–9 years after restoration. In the restored sections, instream microhabitat heterogeneity was higher due to the increased presence of finer substrates, while cobbles and coarse gravel were still dominant. Instream habitat composition did not change between the two sampling events. Areas of restored floodplain were characterized by a more diverse habitat mosaic and by unvegetated bars, vegetated islands and secondary channels. In restored sections, floodplain habitat heterogeneity was maintained 7–9 years after restoration, but vegetated areas increased, while unvegetated bars and aquatic areas decreased. The species composition of all studied groups was more variable over time in restored than non-restored sections. In contrast to benthic invertebrates, the immigration rate of floodplain vegetation and carabid beetle species was higher in restored sections. Assemblage composition of all three organism groups changed over time, with the highest change in carabid beetles and smallest in benthic invertebrates. Restoration changed the abundances of functional response groups, mainly for carabid beetles, by supporting species that indicate increased hydrodynamics and early successional stages. Changes of functional response groups in non-restored and restored sections across time indicated decreased hydrodynamics or hydrological connectivity for all organism groups. Although the response of organism groups differed, our results support the conjecture that restored sections accumulate species and enhance the local species pool.

Influence of vegetation and seasonal flow patterns on parafluvial nitrogen retention in a 7th-order river

We examined whether presence of vegetation and seasonal changes in flow affected N chemistry and denitrification rates within sandbars of a 7th-order sandy alluvial river (Wisconsin River, USA). We addressed these questions with a broad-scale approach of measuring parafluvial water chemistry and denitrification rates in multiple sandbars distributed along a 15-km river reach during summer 2004 and 2005. After recession of spring flooding, parafluvial chemistry in unvegetated bars was characterized by moderate dissolved O2 (DO) and elevated NO3−-N concentrations (>3.5 mg N/L), whereas vegetated bars tended to be hypoxic (<2 mg/L DO) and depleted in NO3−-N (0.2 mg N/L) relative to unvegetated bars and surface water (0.47 mg N/L). As flow declined over the summer, NO3−-N also declined in both bar types, whereas SO42− was relatively constant in unvegetated bars but decreased in vegetated bars. Amendment experiments demonstrated that denitrification was limited primarily by NO3−-N and secondarily by organic C in both bar types, but the strength of this limitation varied over time and was greater in vegetated bars, a result suggesting loss of denitrification capacity. Thus, spatial and temporal patterns of water chemistry and denitrification activity among multiple sandbars indicated that unvegetated bars shifted from N transformers/NO3−-N sources early in the summer to N retainers/NO3−-N sinks as discharge declined, whereas vegetated bars always supported anaerobic processes and probably shifted from NO3−-N to SO42− sinks. We hypothesize that the contribution of vegetated islands to overall riverine N retention is small because establishment of vegetation reduces hydrologic linkages between bars and surface water. Modern changes in the flow regime of the Wisconsin River have increased establishment of riparian vegetation on exposed bars, a pattern suggesting that parafluvial N retention is being reduced while riverine N loading is increasing.

Hydromorphology and land use at different spatial scales: expectations in a changing climate scenario for medium-sized rivers of the Western Italian Alps

The study presents an investigation of the relationships between hydromorphological features and land use in two medium-sized Italian Alpine catchments. The South European River Habitat Survey (SE_RHS) method was applied in the collection of hydromorphological data. Five different spatial scales ranging from catchment to site scale were considered for the quantification of land use. Separate Principal Component Analyses for grouped data sets were run on hydromorphological variables related to different spatial scales and the type of information provided. The analyses demonstrated that the most important gradients are related to channel stability. Unstable river reaches, characterized by a significant presence of unvegetated bars, were predominantly related to natural land use. On the contrary, more stable river reaches, characterized by the presence of vegetated bars, were found to be related to anthropogenic land use. High turbulence flow types appeared to be associated with urban and industrial land use. The climate scenario predicts a general increase in temperature, a slight decrease in precipitation, a more dynamic discharge regime and an increase in extreme events, all of which may produce considerable modifications to present agricultural and river management practices. A conceptual model based on the situation observed in the studied catchments is presented, that can be used to infer possible land use changes, during climate variation, in areas adjacent to rivers.

Input, storage and distribution of large woody debris along a mountain river continuum, the Drôme River, France

Large woody debris (LWD) input, storage and distribution were studied along the Drôme River, a French Alpine river with an active shifting channel and a well-developed riparian forest. LWD input from the floodplain is low: 669.6 mg year −1 between 1948 and 1971 and 569.3 mg year −1 between 1971 and 1991. Based on estimates of average LWD mass per study plot, a range of 766–2122 mg year −1 of LWD were stored within the active channel (e.g., unvegetated bars and low-flow channel; 60 km course covering 492 ha). LWD accumulations are mainly observed on gravel bars at a limited number of preferential sites. They are relatively rare and are randomly distributed in the low-flow channel. Consequently, LWD location is mainly associated with the decrease of flow level in shallow sectors. In the low-flow channel, LWD stop-en-route is primarily caused by in-channel structures such as boulders or vegetated islets. General geomorphological factors (e.g., pattern, slope, etc.) provide less explanation of LWD distribution. Moreover, the residence time of LWD accumulations on the Drôme River is short (LWD storage=1.3–3.7 times the annual LWD input from the floodplain) and their morphogenic role is negligible: few of the accumulations are buried or characterized by vegetation shoots and associated pools.