Rivers have been channelized, deepened and constrained by embankments for centuries to increase agricultural productivity and improve flood defences. This has decreased the hydrological connectivity between rivers and their floodplains. We quantified the hydrological regime of a wet grassland meadow prior to and after the removal of river embankments. River and groundwater chemistry were also monitored to examine hydrological controls on floodplain nutrient status. Prior to restoration, the highest river flows (∼2 m 3 s -1 ) were retained by the embankments. Under these flow conditions the usual hydraulic gradient from the floodplain to the river was reversed so that subsurface flows were directed towards the floodplain. Groundwater was depleted in dissolved oxygen (mean: 0.6 mg O 2 L -1 ) and nitrate (mean: 0.5 mg NO 3 --N L -1 ) relative to river water (mean: 10.8 mg O 2 L -1 and 6.2 mg NO 3 --N L -1 , respectively). Removal of the embankments has reduced the channel capacity by an average of 60%. This has facilitated over-bank flow which is likely to favour conditions for improved flood storage and removal of river nutrients by floodplain sediments. Hydrologie du système rivière-lit majeur d'un cours d'eau endigué de plaine sur la craie et réponse initiale à la suppression des diguesRésumé Depuis des siècles, les cours d'eau ont été canalisés, sur-creusés, et endigués pour accroître la productivité agricole et améliorer la protection contre les crues. Il en a résulté une baisse de la connectivité hydrologique entre ces cours d'eau et leur lit majeur. Nous avons quantifié le régime hydrologique d'une prairie humide avant et après l'élimination de digues la séparant du cours d'eau adjacent. Les compositions chimiques du cours d'eau et de la nappe phréatique ont également été suivies afin de caractériser les interactions entre hydrologie et statut trophique du sol au niveau du lit majeur. Avant restauration, même les plus hauts débits (∼2 m 3 s -1 ) étaient contenus dans le lit mineur par les digues. Dans ces conditions, le gradient hydraulique, dirigé habituellement du lit majeur vers le cours d'eau, s'inversait de telle manière que les écoulements souterrains s'effectuaient en direction du lit majeur. L'eau du sous-sol était plus pauvre en oxygène dissous (moyenne : 0,6 mg O 2 L -1 ) et en nitrates (moyenne: 0,5 mg NO 3 --N L -1 ) que le cours d'eau (moyenne: 10,8 mg O 2 L -1 et 6,2 mg NO 3 --N L -1 , respectivement). L'effacement des digues a réduit la capacité du lit mineur de 60% en moyenne. En facilitant les débordements, les travaux de restauration devraient conduire à un stockage plus important en période de crue et à une épuration de la rivière par stockage des éléments minéraux dans les alluvions.

We examined the hydrologic controls on nitrogen biogeochemistry in the hyporheic zone of the Tanana River, a glacially-fed river, in interior Alaska. We measured hyporheic solute concentrations, gas partial pressures, water table height, and flow rates along subsurface flowpaths on two islands for three summers. Denitrification was quantified using an in situ 15 NO 3 -push-pull technique. Hyporheic water level responded rapidly to change in river stage, with the sites flooding periodically in mid-July to earlyAugust. Nitrate concentration was nearly 3-fold greater in river (ca. 100 lg NO 3 --N l -1 ) than hyporheic water (ca. 38 lg NO 3 --N l -1 ), but approximately 60-80% of river nitrate was removed during the first 50 m of hyporheic flowpath. Denitrification during high river stage ranged from 1.9 to 29.4 mg N kg sediment -1 day -1 . Hotspots of methane partial pressure, averaging 50,000 ppmv, occurred in densely vegetated sites in conjunction with mean oxygen concentration below 0.5 mgO 2 l -1 . Hyporheic flow was an important mechanism of nitrogen supply to microbes and plant roots, transporting on average 0.41 gNO 3 --N m -2 day -1 , 0.22 g NH 4 + -N m -2 day -1 , and 3.6 g DON m -2 day -1 through surface sediment (top 2 m). Our results suggest that denitrification can be a major sink for river nitrate in boreal forest floodplain soils, particularly at the river-sediment interface. The stability of the river hydrograph and the resulting duration of soil saturation are key factors regulating the redox environment and anaerobic metabolism in the hyporheic zone.

We examined the chemical, morphological, and anthropogenic controls on winter‐oxygen biogeochemistry in ice‐covered lakes and reservoirs on the North Slope of Alaska. We measured dissolved oxygen (DO), solute concentrations, water depth, and ice thickness at three natural thaw lakes and four reservoirs (flooded gravel mines) for two winters. In all seven study sites, DO concentration and pH decreased with depth, and temporally through the winter (November to April). DO concentration was four to six times greater in the deeper reservoirs (8‐13 mg/l) compared with shallow natural lakes (ca. 2 mg/l). Lakes and reservoirs with high dissolved organic carbon (DOC) concentration were susceptible to large decreases in oxygen over the winter. DO concentration differed markedly between years, but was not attributed to changes in water‐use or winter water‐chemistry. Alternatively, we suggest that dissolved oxygen concentration was lower during freeze‐up, possibly associated with higher lake‐productivity during the summer. Our results suggest that current water‐use practices on the North Slope of Alaska caused little to no change in DO concentration over the winter. In particular, considering the high pumping activity and shallow depth, lakes with low DOC concentration (≤6 mg/l) showed strong resilience to change in chemistry over the winter. We suggest that both lake and reservoir depth, and DOC concentration are key factors influencing oxygen consumption in ice‐covered arctic lakes and reservoirs.

Channelization and embankment of rivers has led to major ecological degradation of aquatic habitats worldwide. River restoration can be used to restore favourable hydrological conditions for target species or processes. However, the effects of river restoration on hydraulic and hydrological processes are complex and are often difficult to determine because of the long-term monitoring required before and after restoration works. Our study is based on rarely available, detailed pre-restoration and post-restoration hydrological data collected from a wet grassland meadow in Norfolk, UK, and provides important insights into the hydrological effects of river restoration. Groundwater hydrology and climate were monitored from 2007 to 2010. Based on our data, we developed coupled hydrological/hydraulic models of pre-embankment and post-embankment conditions using the MIKE-SHE/MIKE 11 system. Simulated groundwater levels compared well with observed groundwater. Removal of the river embankments resulted in widespread floodplain inundation at high river flows (>1.7 m 3 s À1) and frequent localized flooding at the river edge during smaller events (>0.6 m 3 s À1). Subsequently, groundwater levels were higher and subsurface storage was greater. The restoration had a moderate effect on flood peak attenuation and improved free drainage to the river. Our results suggest that embankment removal can increase river-floodplain hydrological connectivity to form a more natural wetland ecotone, driven by frequent localized flood disturbance. This has important implications for the planning and management of river restoration projects that aim to enhance floodwater storage, floodplain species composition and biogeochemical cycling of nutrients.