Complex sentence: They are now only used for pleasure cruising; this is restricted to the summer months, for between October and April, the water levels are maintained at a low level, so that there is scope to deal with high volumes of rainfall.
Split into simpler sentences: They are now only used for pleasure cruising, and this is restricted to the summer months. Between October and April, the water levels are maintained at a low level, so that there is scope to deal with high volumes of rainfall.