Owing to severe chemical pollution (metobromuron and diazinon), reported on the 25’th of april 1996 in the river Maas off Maastricht, a survey was held (12, 13 july 1996) in order to determine the impact of the relative short-lived toxic wave on the river mollusca. Three of the four sampling sites between Maastricht and Itteren (10 km.) showed however no significant change in the number of species with respect to the number of species collected during a survey held before the pollution (Klink & Bij de Vaate, 1994) . On the other hand, the sampling site in front of the weir of Borgharen showed an unexpected and striking increase in the number of species (Fig.1) and individuals (Table 1 and 2). Due to the high level of dissolved oxygen in the water, caused by the waterfall of the weir, this site creates a pre-eminently favourable habitat for river mollusca and therefore may be interpreted into a shelter place. Probably during long-term pollution as well and particularly for Bivalvia because of the absence of (polluted) sediment due to the locally strong waterturbulence. From literature we know that areas with seepage in the river branches give also some protection at the time of heavy pollution (Wolff, 1968,1970; Kuiper & Wolff,1970). The restpopulations, surviving in the shelters mentioned before, are able to spread out again when the circumstances turns favourable i.e. water-quality improvement and hence, a new, relative clean (thin) layer of sediment. According to this consideration we suppose that most of the pollution sensitive Bivalvia species (Unionidae and Sphaeriidae) probably never went away from the Dutch rivers at the time of the heavy pollution in the sixties and seventies, but were hiding in the small, well oxygenated and seepage locations. This is probably the root for the present re-colonisation of the rivers by mollusca. At present, most of the original Bivalvia species are back again (Wallbrink, 1992, 1993). However, competition due to immigrants (e.g. Corbicula) cannot be ruled out and might reduce the potentional survival of the original species. Fast and easy reproductive species are in advance and are able to settle first in the unoccupied niches after the pollution. We assume however that things may take a turn after a couple of strong winters. Nevertheless, due to the small scale of the shelter areas the species become very vulnerable at the time of pollution. Nowadays the populations of pollution sensitive species in Holland, like Pseudanodonta complanata and Sphaerium rivicola, are slowly increasing again but in the surrounding countries (Germany and Belgium) they are still decreasing. From the point of view of nature management (international responsibility) it is therefore recommended to map the shelter areas for possible future conservation.