Evidence of Taxonomic and Functional Recovery of Macroinvertebrate Communities Following River Restoration

River ecosystems have been heavily degraded globally due to channel hydromorphological modifications or alterations to catchment-wide processes. Restoration actions aimed at addressing these changes and restoring ecological integrity are increasing, but evidence of the effectiveness of these actions is variable. Using a rare 7-year before-after-control-impact (BACI) study of restoration of a lowland groundwater-fed river in England, UK, we explore changes in the macroinvertebrate community following the removal of impoundments and channel narrowing to aid restoration of physical processes. Restoration activity prompted significant taxonomic and functional responses of benthic invertebrate communities in the 4 years post-restoration. Specifically, significant gains in taxonomic and functional richness were evident following restoration, although corresponding evenness and diversity measures did not mirror these trends. Restoration activities prompted a shift to more rheophilic taxa and associated traits matching the physical changes to the channel and habitat composition. Temporal changes were clearer for taxonomic compositions compared to the functional properties of macroinvertebrate communities, indicating a functional redundancy effect of new colonists inhabiting restored reaches following restoration. The results highlight the value of long-term BACI studies in river restoration assessments, as well as project appraisals incorporating both taxonomic and functional observations. We highlight the urgent need of such studies to provide evidence to inform effective river restoration strategies to address future changes such as adaption to climate change and the biodiversity crisis.

Channel narrowing by inset floodplain formation of the lower Green River in the Canyonlands region, Utah

The lower Green River episodically narrowed between the mid-1930s and present day through deposition of new floodplains within a wider channel that had been established and/or maintained during the early twentieth century pluvial period. Comparison of air photos spanning a 74-yr period (1940–2014) and covering a 61 km study area shows that the channel narrowed by 12% from 138 ± 3.4 m to 122 ± 2.1 m. Stratigraphic and sedimentologic analysis and tree ring dating of a floodplain trench corroborates the air photo analysis and suggests that the initial phase of floodplain formation began by the mid-1930s, approximately the same time that the flow regime decreased in total annual and peak annual flow. Tamarisk, a nonnative shrub, began to establish in the 1930s as well. Narrowing from the 1940s to the mid-1980s was insignificant, because floodplain formation was approximately matched by bank erosion. Air photo analysis demonstrates that the most significant episode of narrowing was underway by the late 1980s, and analysis of the trench shows that floodplain formation had begun in the mid-1980s during a multi-year period of low peak annual flow. Air photo analysis shows that mean channel width decreased by ∼7% between 1993 and 2009. A new phase of narrowing may have begun in 2003, based on evidence in the trench. Comparison of field surveys made in 1998 and 2015 in an 8.5 km reach near Fort Bottom suggests that narrowing continues and demonstrates that new floodplain formation has been a very small proportion of the total annual fine sediment flux of the Green River. Vertical accretion of new floodplains near Fort Bottom averaged 2.4 m between 1998 and 2015 but only accounted for ∼1.5% of the estimated fine sediment flux during that period. Flood control by Flaming Gorge Dam after 1962 significantly influenced flow regime, reducing the magnitude of the annual snowmelt flood and increasing the magnitude of base flows. Though narrowing was initiated by changes in flow regime, native and nonnative riparian vegetation promoted floodplain formation and channel narrowing especially through establishment on channel bars and incipient floodplains during years of small annual floods.

Differential bank migration and the maintenance of channel width in meandering river bends

Comparison of three bare-earth lidar data sets along 30 consecutive river bends on the Trinity River in Texas, USA, shows that differential migration of river banks in a channel bend counterbalances past bank migrations so that a statistically steady-state channel width is maintained. Two difference maps created from lidar flown in 2011, 2015, and 2017 capture this temporal variability in the relative amounts of inner versus outer bank migration. In 20 of the studied river bends, channel narrowing during 2011–2015 was counterbalanced by widening during the second interval, or vice versa. Only four bends recorded significant (>1%) progressive change in channel width during both measurement periods. Each of these four bends recorded progressive channel narrowing that was connected to floodplain complexity associated with past bend cutoffs or tributaries. Subaerial volumes of sediment deposited on inner banks of bends were smaller for 2015–2017 than for 2011–2015, while erosional volumes associated with the outer banks were similar despite 2015–2017 having had almost twice the number of days under flood conditions. Over time, channel width for the river appears roughly constant because differences in outer and inner bank migration at one time are counterbalanced by compensating differences at a later time. For the Trinity River, this compensation happened over time spans as short as 2–3 yr and would lead to the appearance of invariant channel width at the decadal scale. Tighter river bends with relatively smaller radii of curvature have smaller magnitudes of width change compared to broader bends with larger radii of curvature.

FEATURES OF HYDRAULICS OF THE RIVER SECTORS WITH RIVERBED WINTERING HOLES OF THE IRTYSH IN OPEN WATER PERIOD

The article describes features of the hydraulics of the Irtysh riverbed in the sections of significant fish concentrations -wintering holes - in the open water period. There have been explored the waters of the largest (in area and in depth) Gornoslinkinskaya and Kondinskaya riverbed depressions located in the Uvat and Khanty-Mansi districts of the Tyumen region and Khanty-Mansi Autonomous District, respectively. The bathymetric characteristics of wintering holes were studied using computerized hydroacoustic complex AsCor (Promgidroakustika, Ltd., Petrozavodsk). To create the bottom relief of riverbeds there were used geographic information software programs Surfer 9.0 and Map Viewer 6.0. The study of the species composition of the fish population was carried out conducting control catches with stationary and drift nets. It has been found that, as a result of the combination of features of the riverbed, on the investigated sectors there is formed a complex hydrodynamic and turbulent environment. The holes are located on meanders with coefficients of high curvature of the bend of a channel, there have been found the ranges with narrowing channels at the entrance into the turn. In the process of the channel narrowing there occurs deepening of dynamic river flow axis and eroding of the bottom, which may prevent silting of the wintering holes. Significant differences in the depths cause a compensating current. Due to the bend of the riverbed in the water areas there are formed whirlpool zones, and arising transverse water currents close the surface and bottom streams of the river flow. The heterogeneous hydrodynamic environment of the Kondinskaya depression is complicated by the fact that the Konda flows into the bend of the Irtysh; as a result, there takes place an exchange of river impulses. Thus, when the longitudinal flow velocity is imposed on the transverse flows, there appears a spiral movement of the water masses and vertical vortex structures - whirlpools resulting in optical (turbidity) and turbulent (hydrodynamic) heterogeneity in the water column of the riverbed wintering holes.

Sedimentary-vegetation response to the channel by-pass: A case study of the Danube river

The Gabčíkovo-Nagymaros waterworks construction on Danube river in Slovakia (1977−1992) was followed with major ecological consequences. The water level and flow velocity was decreased in the original by-passed channel in order to keep high water level for shipping in the bypass canal. The decrease in discharge and lowering flow velocity resulted in the channel narrowing and increase in the vertical accretion magnitude in new floodplain pockets along both sides of by-passed channel. What is more, anthropic intervention in form of groynes structures built for channel adjustment before 1992 dramatically accelerated this process. Vegetation colonisation took place on the new floodplain. In this paper, we analysed land cover changes - vegetation succession using aerial photographs taken in 1986, 1996, 2004 and 2013 as well as field surveys. After the flood in 2013, the area of the new floodplain reached 57% (310 ha) of the original channel area.

Flow in a meandering channel

A comprehensive analysis of the pressure-gradient driven flow in a meandering channel has been presented. This geometry is of interest as it can be used for the creation of streamwise vortices which magnify the transverse transport of scalar quantities, e.g. heat transfer. The linear stability theory has been used to determine the meandering wavelengths required for the vortex formation. It has been demonstrated that reduction of the wavelength results in the onset of flow separation which, when combined with the wall geometry, results in an effective channel narrowing: the stream ‘lifts up’ above the wall and becomes nearly rectilinear, thus eliminating vortex-generating centrifugal forces. Increase of the wavelength also leads to a nearly rectilinear stream, as the slope of the wall modulations becomes negligible. As shear-driven instability may interfere with the formation of vortices, the conditions leading to the onset of such instability have also been investigated. The attributes of the geometry which lead to the most effective vortex generation without any interference from the shear instabilities and with the smallest drag penalty have been identified.