Drainage ditches have been dug in peatlands and paludified forests to enhance forest growth in an area of 4.7 Mha in Finland. Because of peat subsidence, bank erosion, sedimentation, and the ingrowth of vegetation ditches deteriorate with time. In this study the shallowing of ditch depth over time was investigated based on a country-wide peatland inventory data measured repeatedly up to four times. Mixed linear models were developed separately for original ditches and maintained ones (cleaned once or twice). After 20 years, the ditches were 20–30 cm shallower than right after the digging. The time since digging was the most important variable explaining the shallowing for both original and maintained ditches. Other variables explaining the ditch shallowing were the digging method (excavator, plough), ditch bed slope, location, and peat layer thickness. The average development of the maintained and original excavator ditches was very similar. The results can be used in assessing decision making concerning ditch cleaning.

Keywords: forest drainage, ditch depth, ditch network maintenance, modelling, peatland