Gravel augmentation has become common practice to mitigate the effects
of decline in upstream sediment supply in gravel-bed rivers. The success
of such rehabilitation schemes relies partly on the monitoring strategy
and efforts. When long-term monitoring is lacking, some aspects of
rehabilitation initiatives suffer more than others, such as insights
into functions and functionalities of the river system. Despite
temperature being a fundamental parameter determining the general health
of river ecosystems, a limited number of studies have tested whether
gravel-augmentation can aid restoring thermal functions. With the help
of airborne thermal infrared (TIR) imagery, this paper explores the
potential positive feedbacks through the monitoring of gravel
augmentation actions, of different magnitude, taken on 3 rivers of the
Rhône basin in France. A specific trajectory-based
Before-After-Control-Impact (BACI) framework using simple indicators,
combined with a TIR-based Control-Impact strategy, was designed to
assess the success of thermal function restorations based on dynamic
fuzzy references. Results indicate that restoring forms is not
sufficient to restore thermal functions. The control-impact strategy
shows limitations in the sense that two neighbouring reaches can display
similar planform characteristics but different thermal functions; what
is observed in a control reach should not necessarily be expected
following rehabilitation. When assessing thermal processes, a
before-after strategy is needed to either serve as a target or help
define an adequate target in accordance with changes in the catchment
and channel adjustments and responsiveness. We therefore recommend a
trajectory-based BACI assessment to identify current biogeophysical
conditions within which rehabilitation can be assessed. From a technical
perspective, airborne TIR proved to be useful to rapidly map surface
temperature over dozens of kilometres at high resolution, and can be
advocated as a powerful tool to monitor and diagnose thermal functions
of gravel-bed rivers. With an increasing number of rehabilitation
schemes, and increasing pressure of global changes on rivers, we suggest
that monitoring of water temperature, even with simple but well-designed
sampling strategies, becomes a routine part of river rehabilitation
projects.
Long-term bedload mobility in gravel-bed rivers using iron slag as a tracer
