The Tuhkajoki is a small stream flowing into Lake Jormasjärvi from the west. Beginning in Lake Kolmisoppi, the steep profile of the Tuhkajoki drops nearly 35 metres over the course of two rapids. The Niskalankoski rapid begins immediately at the lake outlet. A stretch of slower water follows, with the next steep section beginning at Tuhkakylä village and churning down for a length of two kilometres. For the rest of its course, the river flows in a succession of mild currents towards Lake Jormasjärvi.
White-water fishing is concentrated near the bridges, the most frequent catch being a stocked trout in excess of one kilo. This five-kilometre small stream has many peaceful stretches, especially in the upper reaches where there is no habitation. Here, natural-stock grayling and trout are commonly caught. The almost brook-like Tuhkajoki is three to five metres across, with the best fishing found in the early summer.
The length of the river is five kilometres, with a drop in height of 33.4 metres.
Rapids from upstream to downstream:
- Length: 700m
- Elevation differential: 13.6m
- Average flow: 1.2 m³/s
- Length: 1600m
- Elevation differential: 18.0m
- Average flow: 1.3 m³/s