Freshwater ecosystems are amongst the most threatened in the world. They have been -and still are- altered by a wide range of human activities; from damming, dredging, mining and diversion, to water abstraction, pollution and over-exploitation of sediment and biota. Later recognition of the important ecosystem services provided by rivers, wetlands and floodplains, triggered a need for the restoration and rehabilitation of degraded freshwater systems. The protection and conservation of endangered species has been a particular focus of efforts. The River Ehen, in Northwest England, is one such restoration initiative. The river is naturally regulated by a post-glacial lake, but its exploitation for water abstraction led to the construction of a weir and the diversion of a headwater tributary (several decades ago) that previously discharged just downstream from the lake outlet. Ben Gill is a first order ephemeral tributary whose diversion halted the natural transfer of water and sediments into the main-stem Ehen. This had an impact on the natural behaviour of the system and conditions in the river deteriorated. In the early 2010s, concerns were voiced about the consequences of habitat deterioration for the survival of the freshwater pearl mussel (Margaritifera margaritifera L.). This endangered species is present in high densities in the upper Ehen; in fact, the Ehen hosts the largest remaining population of England. In response to these concerns, the Environment Agency decided to review the water abstraction licence and restore natural sediment dynamics in the river by reconnecting the ‘lost’ tributary, Ben Gill. The objective of the restoration is rather simple in its aim, but complex in its application: restore physical habitat to sustain the pearl mussel population. This thesis focuses on evaluating the changes to fluvial processes and resulting physical habitat in response to the restoration actions and assessing whether these were in line with what we know is suitable for pearl mussels. It sits on the back of preliminary efforts to characterise the pre-restoration conditions of the River Ehen. The reconnection of Ben Gill is the centre-piece of this restoration, so it was crucial to monitor its evolution in order to assess the volumes and frequency of sediment delivery into the Ehen. Because the stream is ephemeral, it was possible to capture its topography under dry conditions, using Structure-from-Motion (SfM) photogrammetry. This technique is becoming common-place in fluvial geomorphology, but this thesis presents the first example of its application within a river restoration context. Aerial images were collected from a small UAV and used to generate 3D models of the channel topography. Successive topographic models were then compared to determine the volumes of erosion and deposition within the channel and infer a minimum volume of export to the Ehen. Additionally, given the relatively small size of the channel (300 m long) and the possibility to install a dense network of ground control points (necessary to properly geo-reference SfM photogrammetry outputs), it was possible to test and determine the potential of using a camera with a fish-eye lens, often criticised as unsuitable for SfM photogrammetry. The results showed that SfM photogrammetry can be applied using a fish-eye camera, given appropriate calibration, assessment of errors and bespoke ground control point network. Model accuracy was consistent over time and the quality high enough to accurately capture the geomorphic evolution of the channel. Confidence in the results was improved by the unexpectedly large scale of the changes observed. These changes meant that large volumes of sediment were exported from Ben Gill and deposited in the Ehen from the early days of the opening of the channel. As part of monitoring related to the reconnection, the Environment Agency installed a series of turbidity meters in the Ehen which, after empirical field-based calibration, provided time-series of suspended sediment concentrations. It was thus possible to assess the changes in the dynamics of suspended sediments in relation to the reactivation of its headwater subcatchment. By comparing 2 years prior to and 2 years following the reconnection, it was possible to characterise the role played by this small, ephemeral but highly dynamic tributary in driving suspended sediment dynamics in the regulated Ehen. On average, the suspended sediment load increased by 65% following the reconnection (i.e. for an increase in catchment size of 1.2%), with most transport occurring through short but intense flow events. This was of particular interest from a geomorphic process perspective, since most studies of ephemeral streams are concentrated in arid or Mediterranean regions; little is known about their relative influence in temperate climate regions. A significant finding from a management perspective is that the increased suspended sediment loads in the Ehen –if left unchecked– may threaten mussels and counter other positive changes resulting from the reconnection. Although mussels can cope with short-lived high suspended sediment events, they are highly vulnerable to the deposition and accumulation of fines on the riverbed. This is particularly true for the juveniles that live buried in the top layer sediment for the first few years of their life. Thus, in-channel fine sediment storage was monitored for over 2 years in the Ehen to assess the effects that the increased suspended sediment loads had on mussel habitat. Analysis of the timing of flows in the Ehen compared to those in Ben Gill revealed that in-channel storage was controlled by the degree of synchronicity between the two streams. High deposition occurred when Ben Gill was connected but flows did not spill over the weir, while the removal of fines was only observed for very high discharges in the Ehen. As with suspended sediment loads, the storage of fines in the riverbed increased following the reconnection. The geomorphic adjustments in the Ehen in response to the newly restored delivery of coarse sediment from Ben Gill was assessed by looking at changes in grain size distribution, surface particle mobility and cross-sectional topography. A large fraction of the material exported from Ben Gill accumulated at the confluence, forming downstream a continuously growing depositional bar. Using the same SfM photogrammetry technique used for Ben Gill, the growth and role of the depositional bar as a transient storage were assessed. It appears, at least in the first two years following the reconnection, only a relatively small fraction of material deposited on the bar is actually re-mobilised and conveyed downstream in the Ehen; most deposits remain on the bar which consequently has kept growing in size. Nevertheless, topographic surveys have confirmed visual observations of deposition in the first 300 m downstream from the confluence, notably a new lateral bar on the true right bank. Other areas have been subject to erosion. Grain size distribution has also changed in the most active morphological units, with an overall trend towards a fining of the bed texture. Surface particle mobility has increased since the reconnection, but still remains rather limited despite the occurrence of very high flows during the period of this study. Nevertheless, this increase in mobility is significant in the two most active morphological units and, along with the observed changes in topography, bode well for a continued improvement of conditions in the future. The hydraulic and geomorphic changes observed in the Ehen as a result of flow regulation and the diversion of Ben Gill are thought to have facilitated excessive growth of algae on the river bed. To investigate this, a 13-month study of the factors potentially influencing algal growth, including the abundance of grazing invertebrates, was undertaken. Monthly surveys were undertaken at two sites in the Ehen and two in Croasdale Beck, an unregulated tributary of the Ehen. This work emphasised the different temporal dynamics of algal growth in the Ehen compared to Croasdale. Algal abundance, represented using total chlorophyll-a, varied according to the magnitude of bed disturbance in the period preceding sampling dates. Invertebrate communities differed between the Ehen and Croasdale, largely reflecting changes in the abundance of different functional feeding groups. Generalised Estimating Equations suggested some of the variation in total algal abundance (chlorophyll-al) could be accounted for by grazer abundance. However, ordination-based analysis of the composition of the algal community suggested more complex interactions, with environmental controls (flow hydraulics, nutrients, bed stability) more important than simply grazing pressure. The reconnection of Ben Gill to its main-stem Ehen is a unique example of non-invasive catchment-scale restoration initiative of an upland river. In this sense, it differs from some rehabilitation projects whose actions are limited to things such as artificial gravel augmentation, the installation of deflectors or other in-channel features, or in some cases the ‘landscape gardening’ of a target reach. The focus of the Ehen project is on restoring fluvial processes which, in turn, will help achieve the objective of improving physical habitat for the pearl mussel. Although proper assessment of the response of the pearl mussel population to the restoration will require more time and investigation, the primary goal of the reconnection has been achieved; the natural delivery of coarse sediments has been reactivated and material is being conveyed downstream the upper Ehen, creating new geomorphic features and increasing habitat diversity. As is the case with any restoration or rehabilitation project, there has been uncertainty and unpredictability as to how the tributary might behave and how the river might respond. The primary and most worrying manifestation has been the marked increase in fine sediment loads. Given the sensitivity of the focal species, these high loads have been a cause for concern for the government agencies and conservation groups involved in the project. This has raised interesting questions about the possible need for intervention, though for the moment the decision has been made to simply monitor how the situation evolves. This and all other elements of the reconnection have ensured that the restoration of the Ehen has been an invaluable learning experience from a management perspective, and especially regarding the role of monitoring to help assess river response and the achievement of target objectives.