Chalk stream restoration: Physical and ecological responses to gravel augmentation

Abstract

To mitigate the morphological and ecological impacts of direct (e.g. dredging) and indirect (e.g. damaged river function) sediment loss, gravel augmentation is commonly practiced in river systems globally. Despite this, the effectiveness of this practice remains poorly understood, especially in less often considered systems such as chalk streams which present uncommon conditions (e.g. low stream power, stable flow) and may respond to interventions in ways that differ from systems more commonly studied. This study quantified immediate (0-1 years) and short-term (1-2 years) physical and ecological responses to gravel augmentation at two English chalk stream restoration sites: Home Stream (HS; River Test) and East Lodge (EL; River Itchen). We quantified habitat (depth, velocity, substrate composition), cover of different macrophytes, and macroinvertebrate (before-after-control-impact) abundance and community structure. Restoration reduced depth and increased gravel cover in both sites and decreased the cover of filamentous green algae in HS. Macroinvertebrate communities became more dominated by silt-intolerant taxa, while abundance [HS only] and taxon richness increased 1-2 years post-restoration. Whilst the responses found were generally positive in light of the restoration goals, the effects varied across sites, post-restoration time periods and ecological groups, emphasising the need for the more holistic monitoring of restoration projects considering community-level responses at different sites and systems over ecologically relevant timescales. This will help inform on the generality and longevity of responses and provide the evidence needed to develop sound restoration practice.

Fig 1

(A) Location of study sites on the Rivers Test and Itchen in Hampshire, southern England, United Kingdom, with greater detail provided for (B) Home Stream (HS) 51.07296, -1.51685, and (C) East Lodge (EL) 51.00047, -1.32551. The arrows, black lines and crosses respectively indicate flow direction, extent of control and restored reaches and positions where felled trees were secured. Country and county shapefiles and river network were supplied from [68, 69], respectively. Detailed maps (B and C) were supplied from [70].

Fig 2. River discharge (m³ s^-1) for the Rivers Test (HS) and Itchen (EL) during a study to quantify physical and ecological response to gravel augmentation.

Symbols in grey shaded area show the months during which data was collected at HS (River Test, grey symbols) and EL (River Itchen, black symbols). Flow data was obtained from Chilbolton (River Test) and Highbridge (River Itchen) gauging stations, approximately 10 km upstream and 2 km downstream of the restored sites, respectively [77]. Contains public sector information licensed under the Open Government Licence v3.0.

Fig 3. Changes in physical habitat metrics in HS and EL restored and control sites prior to and 0–1 and 1–2 years post gravel augmentation.

Black bars and boxes indicate median and 25^th and 75^th percentiles, respectively. Whiskers represent minimum and maximum values excluding outliers. Dots show outliers (values > 1.5 x the interquartile range).

Fig 4. Substrate composition in restored and control reaches in HS and EL prior to and 0–1 and 1–2 years post gravel augmentation.

Fig 5. The changes in total macrophyte cover (TMC) and filamentous green algae (FGA), broad leaved (BL) macrophyte and tape grass (m²) in HS and EL restored and control sites prior to and 0–1 and 1–2 years post-restoration with gravel augmentation.

Black bars and boxes indicate median and 25^th and 75^th percentiles, respectively. Whiskers represent minimum and maximum values excluding outliers. Dots show outliers (values > 1.5 x the interquartile range).

Fig 6. The changes in macroinvertebrate metrics in HS and EL restored and control sites prior to and 0–1 and 1–2 years post-restoration with gravel augmentation.

Black bars and boxes indicate median and 25^th and 75^th percentiles, respectively. Whiskers represent minimum and maximum values excluding outliers. Dots show outliers (values > 1.5 x the interquartile range).