There is rafting season even in flat Estonia in early spring when the snow melts and water level is high in rivers. The high water season appears suddenly and it is very short (couple of weeks), thus you really must seize the moment. We use special rafts built for Skandinavian rivers and they are known also among the tour operators in the rivers in the Alps. Jägala river is the best river for rafting in Estonia  and you can get a quick energy shot there.