Interdisciplinary research (archaeological excavations, ground surface surveying, as well as the results of a geomorphological survey and drillings, palynology and radiocarbon dating, and an examination of archival maps and analyses of historical sources) carried out in the vicinity of the Early Medieval stronghold in Nasielsk was focused on the lithological diversity of the sediments filling the Nasielna river valley and its relation to human activity in this area in the past. The study proved that the activity of Early Medieval settlers and the multiphase operation of water mills in medieval and modern times were the main causes impacting valley bottom transformation. The town of Nasielsk is located some 50 km to the north of Warsaw and remains of the Early Medieval stronghold lies in the northwestern part of the town. It measures 85 m in diameter and has a rampart in excess of 2 m in height. According to written sources of the 11th-13th centuries, Nasielsk was a local center of administration (castellany). At the time a part of the town was owned by the Czerwinsk monastery. Archaeological excavations have been conducted in 1976 and in 2001-2006. The conclusion is that the stronghold was a multiphase site operating for more than 400 years. Dendrochronological data from the rampart have dated the construction of the oldest part of the stronghold to the middle of the 9th century. The youngest dendrochronological date falls in the middle of the 13th century (i.e., A.D. 1246). Settlers presumably abandoned the stronghold either in the second half of the 13th or in the beginning of the 14th century. In 1386, the newly-founded town of Nasielsk was given by the Mazovian Duke Janusz the First to the knight Jakusz of Radzanowo. The studied part of Nasielna river valley is filled with organic deposits (peats, organic silts and clays, gyttja) totaling some 1.50 m in thickness. The organic sediments cover fine and medium sands and gravels. Some 50 drillings in the valley bottom next to the stronghold revealed a complicated morphology of the former valley bottom. Geological and geomorphological studies together with enclosed cross-sections have indicated for channel pattern changes in the Nasielna (former exsistance of braided or anastomosing stream). The gyttja-lacustrine sediments (with plant detritus and mollusks at different depths) filling the valley bottom are evidence of a multiphasal operation of ponds in this area (mostly mill and probably fish ponds). The lithological diversification of the sediments in the vicinity of the stronghold has led the authors to assume very short intervals between successive stages of damming-up of the river (possibly from just a few to a dozen or so years). Radiocarbon datings and archaeological data permit a reconstruction of the Nasielna river evolution in eight stages. The application of interdisciplinary methods: geological, geomorphological and archaeological, together with absolute dating has helped to unravel the relations between human activity in the past fourteen-fifteen centuries and the changes of the environment in the vicinity of the Early Medieval stronghold in Nasielsk. It was also possible to study the quality of these changes and the rate at which they occurred, thus providing the data for a reconstruction of the former river valley relief, that is, the landscape of the medieval settlement. 11 Figures.