Abstract
Clogging of gravel-bed rivers is a major issue for fish and
macro-invertebrate habitats as well as for groundwater-river exchanges.
River clogging consists mainly of the deposition and infiltration of
fine sediments within the bed matrix, which is a natural phenomenon but
can be enhanced by human activities. Although there are several methods
for estimating the degree of clogging, quantitative assessment of
clogging remains challenging, not to mention the lack of comparison
between these methods. We therefore implemented three quantitative
methods for estimating clogging (bed material sampling, infiltration
test, and interstitial water sampling), and assessed their effectiveness
and suitability for application as a long-term monitoring solution.
These methods were applied to a natural river bed characterised by high
spatial heterogeneity of clogging. The results show good correlations
between the different methods despite some scatter that can be linked to
limitations of some methodologies in term of sampling itself or sampling
depth. Indeed, sampling of the bed material using a McNeil sampler is
limited to the first 15 cm below the surface, whereas the infiltration
test or interstitial water pumping using a standpipe can be achieved for
a depth deeper than 15 cm only. The infiltration test appears to be
effective for relatively low clogging, while interstitial water sampling
is more efficient for high clogging. The results also show that
dissolved oxygen estimation alone may not be sufficient to accurately
estimate physical clogging, particularly for cases of low clogging.
However, it appears to be a good and simple complementary method to the
interstitial water-sediment pumping method. The infiltration test and
interstitial water-sediment sampling are simple, quick and easy to apply
methods. The two methods are complementary, and suitable for a spatial
clogging assessment, with the possibility of investigating a wide range
of clogging degrees.