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

AbstractGravel 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.