Modern forestry requires a high degree of machine traffic for clear felling and scarification operations. The machines most frequently used are big and the traffic often results in tracks on the forest ground. There are many small streams running through a forest, in connection to logging operations, machines might cross these frequently. These crossings could result in the erosion of fine particulate inorganic matter, which ends up in the stream. The aim of this thesis was to give a general description of small forest streams of order-one in the county of Värmland and to determine the frequency of machine tracks in small streams. This was done by using the database from the "Forest and Water Inventory" in the county of Värmland. It was also determined if the benthic fauna composition had been changed downstream from the tracks, in the parts of the stream that had been affected by erosion. This data was then compared with the parts of the stream that had not been affected. For this purpose, five small streams with similar chemical and physical conditions were chosen for a field study. The most typical stream of order-one was straight lined and with a width less than 0.6 m. Usually the bottom substrate consisted of silt to clay and most of the streams lacked macrophytes. The frequency of large woody debris in the streams was generally low. The results showed that 6.5 percent of all streams had machine tracks. When the riparian zone (0-10 m) was affected by forestry operations, one out of ten streams had machine tracks. In the case of productive forests that had been clear-cut in the surrounding zone (10-60 m) and in the adjacent zone (0-10 m), 21 percent of the streams had machine tracks. The depths of the tracks in the riparian zone of streams subject to tracks were commonly in the range of 1-40 cm. About one-third of the streams with machine tracks was subject to erosion. The degree of erosion depended on the gradient of the riparian zone. When no gradient was evident, there was almost no type of erosion in the stream. With increasing gradient in the riparian zone, the proportion of streams with resulting erosion was increasing. The streams with the steepest riparian zones had the largest proportion of strong erosion. No clear differences could be found in numbers of taxa and individuals, diversity index, pollution index or functional feeding groups between the part of the streams affected by erosion and the part of the streams, which had not been affected. This ...
