Can gravel augmentation restore thermal functions in gravel‐bed rivers? A need to assess success within a trajectory‐based before–after control–impact framework

Abstract

Gravel augmentation has become common practice to mitigate the effects of decline in upstream sediment supply in gravel‐bed rivers. However, the functional aspects of river systems are often left out of rehabilitation monitoring programmes. Despite temperature being a fundamental parameter determining the general health of rivers, a limited number of studies have tested whether gravel augmentation can aid restoring thermal functions. Using airborne thermal infrared (TIR) imagery, this paper explores potential positive outcomes through the monitoring of gravel augmentation actions on three rivers in France using hydro‐morphological indicators within a trajectory‐based before–after control–impact (BACI) framework. This design, combining long‐term geomorphic evolution with TIR‐based CI strategy, indicated that restoring forms was not sufficient to restore thermal functions in their spatial dimension. Despite forms showing various degrees of recovery, the thermal regime of rehabilitated reaches (i.e. temperature gradient, cold‐water patches density, etc.) failed to recover. Nonetheless, hydro‐morphological indices can be used to estimate long‐term evolution of groundwater‐surface water interactions. We emphasise the benefits of trajectory‐based BACI assessment to identify current conditions, understand the past evolution (trajectory) of the system to define the framework within which rehabilitation can be objectively assessed, especially when assessing hydrological (here thermal) processes. From a hydrological perspective, the gap between restored forms and restored thermal processes stems from the alteration of connectivity pathways, which potentially require more time (or efforts) to be recovered than forms do. With an increasing number of rehabilitation schemes and increasing pressure of global changes on rivers, we suggest that monitoring of water temperature becomes a routine part of rehabilitation projects.

Closing the gap between science and management of cold‐water refuges in rivers and streams

“…Armored and clogged streambed sediment (i.e., the erosion-resistant layer of relatively large particles that is established on the surface of the streambed through the removal of finer particles by stream flow) can also impact CWPs by limiting water exchange with groundwater (Wawrzyniak et al, 2016) and reducing groundwater discharge points that can dry out when the water table level lowers as a result of channel incision from mining, logging, grazing, channel straightening, and sediment starvation (Marteau et al, 2022).…”

“…Cold‐water patches fully mix with ambient water at high streamflow in rivers draining large stratified natural lakes that show abrupt wind‐driven drops in river temperature, leading to the loss of discrete CWPs (Lisi & Schindler, 2015) and a reduction in CWRs. Armored and clogged streambed sediment (i.e., the erosion‐resistant layer of relatively large particles that is established on the surface of the streambed through the removal of finer particles by stream flow) can also impact CWPs by limiting water exchange with groundwater (Wawrzyniak et al, 2016) and reducing groundwater discharge points that can dry out when the water table level lowers as a result of channel incision from mining, logging, grazing, channel straightening, and sediment starvation (Marteau et al, 2022).…”

Can we incorrectly link armouring to damming? A need to promote hypothesis-driven rather than expert-based approaches in fluvial geomorphology

“…For instance, in climate change studies the focus has been on attributing human activities to observed long-term changes in climate parameters (surface temperature, precipitation, circulation), as well as extreme events (storms, hurricanes, heat waves) (Zhai et al, 2018). Similarly, in ecological monitoring studies, the search for the quantification of environmental impacts lead to the development of the Before-After-Control-Impact (BACI) design, considered to be a statistically powerful experimental design for environmental impact research (Underwood, 1992;Smokorowski and Randall, 2017) and that is being increasingly applied in fluvial geomorphology (see recent work by Marteau et al, 2022).…”

Geomorphic effects of a run-of-the-river dam in a multi-driver context: The case of the Upper Garonne (Central Pyrenees)

“…A BACI (Before-After-Control-Impact) approach is classically used to assess the effects of river restoration actions (Marteau et al, 2022) but can be appropriate to assess the unit-effect of any driver of change. It relies on the comparison with a "pre-" state (Before), (e.g., before the construction of the Plan d'Arem dam), and with a reach out of the zone of influence of the considered pressure (Control), (e.g., upstream the Plan d'Arem dam).…”