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Analyses of a sediment core from the Great Belt documented the local presence of fluvial deposits. The sediments consisted of silt, clay, sand and organic detritus and they contained macroscopical remains of a mixture of terrestrial, telmatic, lacustrine and fluvial plants and animals. Most noteworthy was the abundance of remains of caddisfly larvae, totalling at least 24 species, which were dominated by fluvial species. Thus, filter-feeding hydropsychids made up approximately 95% out of 1496 identified specimens, the dominating species being Hydropsyche contubernalis and H. pellucidula. The present-day requirements of the recorded species point to a fairly fast-flowing river, at least locally, with areas of stones (some with moss growth) and gravel, but also to more slowly-flowing parts with sand, fine and coarse detritus. This river may have been considerably larger than any present-day Danish river. It appears that the fluvial deposits formed locally in connection with flooding of the area south of the core position. AMS radiocarbon dating shows that the sequence was deposited very rapidly during the early Holocene, about 10650 to 10250 cal. years BP. The occurrence of the lentic Leptocerus tineiformis points to a climate at least as warm as today. [ABSTRACT FROM AUTHOR]