When analyzing the symptoms of eutrophication and water quality in a small retention reservoir Bachmaty, an attempt to assess changes in its trophic condition before and after modernization, was undertaken. The study upon changes in water trophic level was carried out on a small retention reservoir Bachmaty located in Podlasie region on the Orlanka river in the municipality of Dubicze Cerkiewne. Tests of water samples collected from the surface layer of the coastal zone were performed once a month, and the time of the research was divided into two periods. The first period was conducted since April 2007 to March 2008 and since April 2009 to March 2010. The second research period included the time immediately after the reservoir reclamation, which ended in May 2011. The study was conducted since May 2011 to March 2012 and since April 2013 to March 2014. The trophic level of Bachmaty reservoir was also assessed according to the concentration criteria and based on the trophic status indices (TSI) calculated after Carlson’s as well as Kratzer and Brezonik’s. Reclamation of the reservoir, which consisted mainly in removing the sediments, caused a reduction in all TSI values. Analysing the overall trophic level index, i.e. the average of TSI (TP), TSI (TN), and TSI (Chl), water of the Bachmaty reservoir may be classified as eutrophic in individual years of the research, although a gradual decline in the TSI value could be observed directly after the reservoir modernizing. After testing it was found that the formation of the trophic level of Bachmaty reservoir was largely affected by the amount of total phosphorus supplied to the water. Water quality in the reservoirs, and also the content of phosphorus compounds is largely determined by external supply.