ABSTRACT Sedimentation after wildfire is a profound disturbance to the biogeomorphic character of fluvial systems. Despite this significant alteration, field data focusing on the geomorphic processes and bed morphology that form the physical structure of habitat are limited, especially over a longer-term perspective. We report results of detailed field studies following two wildfires in the same steep fluvial system in the southern California Transverse Ranges three decades apart. Substantial channel sedimentation during storms following both fires altered physical elements of habitat, which included bed elevation, step height and spacing, pool depth and spacing, grain-size distribution ( D 84 ), and transport capacity (τ o /τ c ). The postfire sediment deposits buried bedforms, decreased D 84 , and increased τ o /τ c by an order of magnitude. Incision during subsequent storms initiated recovery that depended on the attributes of post-wildfire climate variability that characterize the semiarid Mediterranean-type environment. Step-pool bedforms reappeared or reformed during the decades between wildfires as the relatively fine gravel-sized post-wildfire sediment was transported downstream, thus reestablishing the physical elements of habitat characterizing the dynamic system.

Abstract The Chalk is an unusual karst aquifer with limited cave development, but extensive networks of smaller solutional conduits and fissures enabling rapid groundwater flow. Small-scale karst features (stream sinks, dolines, dissolution pipes, and springs) are common, with hundreds of stream sinks recorded. Tracer velocities from 27 connections between stream sinks and springs have median and mean velocities of 4700 and 4600 m d −1 . Tests to abstraction boreholes also demonstrate very rapid velocities of thousands of metres per day. Natural gradient tests from observation boreholes have rapid velocities of hundreds of metres per day. There is strong geological control on karst with dissolution focused on stratigraphical inception horizons. Surface karst features are concentrated near the Paleogene boundary, or where thin superficial cover occurs, but rapid groundwater flow is also common in other areas. The Chalk has higher storage and contaminant attenuation than classical karst, but recharge, storage and flow are influenced by karst. Point recharge through stream sinks, dolines, losing rivers, vertical solutional fissures, and soakaways enables rapid unsaturated zone flow. Saturated zone networks of solutional fissures and conduits create vulnerability to subsurface activities, and enable long distance transport of point source and diffuse pollutants, which may be derived from outside modelled catchment areas and source protection zones.

Abstract Rivers in karstic environments are known to be greatly influenced by surface water–groundwater interactions, with significant localized inflows during floods from springs, or with losses that can dry up rivers. The Middle Risle River is frequently affected by the development of sinkholes in a chalk karst area (Normandy, France). In the 2010s, two new major sinkholes in the riverbed caused a complete loss of water into the underlying phreatic aquifer, causing the river to dry up over several kilometres. The resulting changes in hydrogeological processes and surface water–groundwater interaction greatly affected water quality, water use and water-dependent ecosystems, causing a political crisis in this river-dependent touristic valley. To understand these phenomena and improve crisis management, the Middle Risle Critical Zone Observatory was set up to enhance monitoring, surveying and/or modelling of groundwater and river levels, river and spring flow, water temperature and conductivity, and ecosystem characteristics (fish, macro-invertebrates and vegetation). The results showed notable impacts on fish, macro-invertebrates and vegetation, some plants proving to be reliable indicators of surface-water–groundwater interaction. The dynamics of local hydrogeological processes were assessed and linked to the measured effects on ecosystems and water supply. Inverse modelling based on an analytical solution of the diffusive wave equation assessed lateral flow during floods, quantifying the spatial–temporal variability of surface-water and groundwater exchanges. It also highlighted the important role of karst zones in both storage and flood-peak attenuation processes, thereby protecting downstream villages against floods.

Abstract The UK, and England in particular, is the stronghold for chalk rivers, streams and wetlands in Europe. A number of sites are recognized as being important for nature conservation and have been designated as such under UK and European legislation. However, as the chalk is also an important aquifer for southern and eastern England, there have been significant impacts on these groundwater-dependent ecosystems from abstraction. Chalk rivers and streams have been used for centuries for mills and water meadows, so impacts have not just occurred in recent times. Intensification of agriculture in the twentieth century has added to the pressure by increasing levels of pollution, especially nitrates, with significant levels now being recorded. However, moves have been made to resolve some of these issues, with investigations into the effects of abstraction and options for reducing these impacts, research into the nature of the chalk aquifer so that it can be modelled more accurately, and assessment made of pollution pathways and their timescales. Associated projects have characterized the ecosystems associated with the chalk in more detail, enabling the mechanism for impacts to be better understood. While the extent of impacts is increasingly understood, action is also being taken to reduce their effects and restore chalk ecosystems.

Abstract The study of the temperature of two rivers in Normandy (France), the Orne and the Touques, between 2013 and 2018 allowed the main controlling factors regulating their thermal regime to be determined. The analysis was conducted by coupling different statistical treatments: linear regression between water and air temperatures, independent component analysis, principal component analysis and a multiple linear regression model. The temperature of the two rivers is mainly controlled by climatic factors but secondary regulation factors are demonstrated to play important roles: runoff for the two rivers and groundwater for the Touques. The influence of the chalk aquifer on river temperature appears to vary seasonally throughout the year, reaching its maximum in the early spring and increasing from upstream to downstream. The coupled use of the different statistical complex methods showed its validity in understanding both the temporal and spatial variations in water temperature and its correlation with the secondary factors, that could not be inferred from a simpler approach based on linear regression. These techniques could be valuable in other areas with rivers sufficiently monitored to determine the controlling water temperature factors and thus their sensitivity to climate change.

Abstract Groundwater of the unconfined Chalk aquifer in Champagne-Ardenne (NE France) is contaminated by perchlorate ( ClO 4 − ), a persistent water-soluble anion. The Chalk aquifer is a crucial water resource of the region, with complex hydraulic properties. The presence of ClO 4 − is of concern due to its potential adverse human health effects. In France, three sources of ClO 4 − contamination are suspected: industrial, military and agricultural. Both a comprehensive understanding of hydrogeological characteristics of the aquifer, and a sound knowledge of sources and behaviour of ClO 4 − in groundwater are required to allow the sustainable use of this groundwater resource. From data acquired during hydrogeological, geochemical, isotopic ( ClO 4 − ) and groundwater age (CFCs and SF 6 ) studies, and historical investigations in a study area located east of Reims, a conceptual model of Chalk aquifer function and ClO 4 − transfer has been established. High spatio-temporal heterogeneities in the unconfined Chalk aquifer are discussed. Different correlations between ClO 4 − , major ions and groundwater-level fluctuations are shown and interpreted, highlighting main factors governing the Chalk groundwater geochemistry and ClO 4 − transfer mechanisms, including water-level fluctuation, groundwater residence time, thickness of the unsaturated zone, superficial formations, distribution of fissure networks, aquifer–river relationships, origin and location of ClO 4 − in soil and human activity.