Large amounts of water can be tightly held in the interspace between the clay particles.
Smaller clay particles and a high calcium content lower the sintering point.
In other words, the clay particles are now in a state of colloid suspension.
As a result, clay particles do not form a stable emulsion and can be readily separated from the water.
Also at this point a lot of "bridging" between clay particles occurs so that the whole mess is a rock, more or less.
So now the clay particles are pushing one another apart.
Lessivage is a kind of leaching from clay particles being carried down in suspension.
Otherwise a layer of clay particles under the usual fine gravel will suffice.
The water has little mineral sediment and few suspended clay particles.
The presence of clay particles is not a major factor in the pollution of the river.