Olst-Wijhe is dominated by river
IJssel, which partly flows through it and partly forms the border between the provinces Overijssel and Gelderland. Although this border largely runs along river IJssel, there are two left-bank
exclaves opposite the towns of Olst and Wijhe, in which the villages of Welsum and Welsumerveld (south exclave) and Marle (north exclave) are situated. There are
cable ferries between Olst and Welsum and between Wijhe and Vorchten in the municipality of
Heerde (there is no direct connection between Marle and the rest of Overijssel). Olst-Wijhe is part of the low-lying
Salland region. The one metre above mean sea level
contour line runs roughly parallel to the river; the municipality's interior is characterised by flat or gently rolling grasslands and woodlands at about 10–15 metres above mean sea level. Apart from river IJssel, several more streams flow through the municipality's territory. They are called
wetering ("drainage waterway") and are amalgamations of natural streams gradually
channelised from the 18th century onwards and specifically dug canals . From west to east, these are Zandwetering, Soestwetering, Groote Vloedgraven, Oude Wetering and Nieuwe Wetering. All these streams flow in a south–north direction and come together at one point or other to eventually form the
Zwarte Water river near the city of
Zwolle. On the left side of river IJssel, near the exclave of Welsum, there are several more
wetering streams, all but one of them on the Gelderland side of the border; the exception being the easternmost of these, the Terwoldsche Wetering, which forms the border between Olst-Wijhe and Voorst. ==Population centres==