It this paper the evaluation of drainage devices which are working at agricultural land of Podlaskie province and at drainage object of Lower Supraśl Valley was made. Their functions in production of vegetable biomass and in natural environmental protection was indicated. Detailed ditches drainage fulfill an important role in water balance in agricultural areas and result adjusting drainage weir in dehydration and irrigation. Meliorative devices were described and evaluated on a large (1153 ha) object of Supraśl Valley. The object has an important economic function (irrigation meadow and pasture) and natural functions (maintaining the diversity of typical flora) of post-bog meadow. Ditches do not assure proper (suitable) moistening on this object of the length of 125,6 km, because they are shallow, silted with slime and they lack proper bottom decreases. 75% of water damming up were working correctly, but the remaining did not grant their function. Drainage culvert enabling communication were proficient in 80%. Lack of their systematic conservation is the reason of low proficiency of detailed drainage devices, it result from lack of regulations in this range.