Naturally, rivers posses a gravel bed with many small crevices in which numerous organisms can be found (Interstitial pore system).
The river mussel spends its first years in this substrate. However, the influx of fine sediments clogs up the crevices so that fresh water is no longer flowing through the interstitial spaces to provide oxygen to the river mussels and to other organisms. The gravel substrate system will be damaged enduringly.
Dams in the upper rivers prevent the discharge of rubble into the lower reach. Thus the river beds are further deprived of gravel.