In this paper given are basic characteristics and projected functions of the DTD (Danube-Tisza-Danube) Hydrosystem (Yugoslavia), then types, origin and some characteristics of muds - sediments in the hydrosystem channels. It is estimated that in hydrosystem channels gets in averagely about 1.28x10**6 tons of deposits annually, and of that amount in channels are settled about 0.64 x 10**6 cubic meter annually. In 1980-1996 period, it is mudded from channels of the main channel network averagely about 0.19 x 10**6 cubic meter/year, and in 1991-1996 period, usually only about 0.056 x 10**6 cubic meter/year, i.e. about 10 of average amounts which are deposited during the year. Also explained are numerous undesirable influences of muds - sediments on water quality in channels, as well as functions of the DTD (Danube-Tisza-Danube) Hydrosystem in general. At the end, measures for gradually improving conditions that are influenced by muds - sediments are, proposed.

During the period 1990-1997, the content of 11 trace metals (Fe, Cr, Co, Al, Mn, Cd, Cu, Ni, Pb, Zn, Sr) was studied in muscle and other organs of fish from DTD (Danube-Tisza-Danube) channel system (Yugoslavia). According to obtained and analysed data, the trace metal bioaccumulation can possibly act as enviromental indicator in biomonitoring. DTD (Danube-Tisza-Danube) Channel System can, therefore, be classified as the 3rd class, concerning analysed trace metal content.

On the basis of the enzymatic (phosphatase) activity and microbiological analysis as well, the influence of the oil refinery "Vital" - Vrbas (Serbia, Yugoslavia) wastewater to the recipient DTD (Danube-Tisza-Danube) canal water from the microbiological point of view was determined. In addition the efficiency of the factory's waste water purification plant was estimated. The obtained results of the complex microbiological-enzymological analyses indicated a good water quality at the point where the refinery's water supplies plant is situated (site 1). Very low water quality was determined downstream (site 4) and upstream (site 2) from the inflow of the refinery's wastewater's inflow. The later results might indicated the influence of an unknown pollutant in the region. However, the obtained O/H index value suggests that the self-purification ability of the canal waters is still preserved.