The river Timok with its watershed area is one of very imperilled running water on the territory of Serbia (Yugoslavia). By several years investigations of colonies bottoms (phytobenthos, microzoobenthos and macrozoobenthos) and plankton we have found the following: great diversity and specific phytobenthos and bottom fauna (frequency of certain taxa is considerably larger in relation to the other areas, and new taxa have been recorded). Stated specific structure of phytobenthos and bottom fauna have considerably reflected to right estimation of water quality of tested area.

Problems encountered in evaluating surface water quality arise from the requirements of relevant regulations that these evaluations should be made in the last favourable hydrologic conditions during design flow rate (required minimum flow). However, the major part of measurements of water quality parameters is not performed under such conditions and the result must be corrected for the conditions under required minimum flow. Two possible approaches to solving this problem are presented in this paper and explained using an example of the Juzna Morava river basin up to the Station Korvingrad, Serbia (Yugoslavia). This basin is unhomogeneous regarding its hydrologic characteristics and mass flow of pollutants. The results of analysis are presented in the cartography form using the GIS technology.

The aim of this paper is to survey and point to the critical parts of this typically valley river in the chosen period of time, which is, because of its low flow, small turbulence and exceptional social and industrial significance, very susceptible to ecological catastrophes. This paper contains the elaborated state of water quality in Yugoslav part of the river Tisza, during two periods of observation, with usual (1988-1991) and specific (1992-1995) river ecosystem exposure to pollutants. The average annual data of concentration for characteristic parameters in the monitoring stations were compared with method of multiobjective iterative compromise optimalization (MICO) of the influence that different categories of polluters have on the river ecosystem. On the basis of analyzed concentrations of selected parameters and results obtained with method of MICO, it is possible to conclude that the improvement in quality of Yugoslav part of the river Tisza has occurred during the period of 1992-1995.

There were estimated of water quality of the river Banja and its trubutary Pocibrava, Serbia (Yugoslavia), during the period 1991-1994 by seasonal investigations of benthos fauna composition as bioindicators. The results have shown higher diversity of the Pocibrava bottom fauna community (66 taxa) than in the river Banja (53 taxa) where it was more uniform. Both water flows were receiving some organic loading and by the use of macrobenthos indicator organisms it was concluded that they belong to beta-mesosaprobic water quality class.

The river Tara (Montenegro, Yugoslavia) is under the treatment of the international (UNESCO) and domestic legal protection, so the prescribed quality of its water is the 1st class. Permanent measurement of the water quality of this river shows that due to the influence of the human factor, its quality has been endangered. Examination of the data collected through many years at all the measurement station along its current shows that the quality of water and pollution estimate have changed.

In this paper, the results of statistical analysis of then years water quality data of Tamis river at the Romanian-Yugoslav border ("Jasa Tomic" Control Station) are presented. Following changes of analyzed water quality parameters at the Romanian-Yugoslav border a strong trend of deterioration has been observed during analyzed period. Because of periodical accidentally high organic content (COD, BOD) and concentration of ammonia and organic nitrogen in river water, it is concluded that upstream discharging of farm waste waters was the main reason of deterioration of water quality along the Yugoslav part of Tamis river.

Low flows are a good measure of the waste water recipient self-purification capability. The regional statistical analysis is an objective way of assessment of the required low flows. This paper presents the principles and key phases of the regional statistical analysis. The advantages of this method are a reduction of the outliers influence and the assessment of low flows on ungauged streams and stream profiles. The method is illustrated with the results obtained by the required low flows regional statistical analysis in Serbia (Yugoslavia) with 59 hydrological gauge stations in the scope, which data has the same length of 39 years, from the year 1956 to 1994.

During hydrobiological investigations of the river Dulenska (Serbia, Yugoslavia) in June 1996, algological samples were taken at this river. In the algae community are found 34 taxa from two divisio: Bacillariophyta (28) and Chlorophyta (6). While qualitative composition of the algae colony was relativelly uniformed quantitative one was changeable along the course of the Dulenska river. By saprobiological analysis, it was found that the quality of water was changing along the course of the river. At the upper and middle course of river the water belonging to the second class. At lower course of the river (below Rekovac) water quality was getfing worse and it belonging to the third class.

Drought characteristics (minimum flows, drought duration and volume deficit) of the Moraca river near Podgorica, Montenegro (Yugoslavia) are analyzed using the peaks over threshold method based on renewal processes. It is also shown that volume deficit and drought duration are highly correlated.

The paper provides experimental results of testing the content of heavy metals, and other harmful and dangerous matters of the Sava river sludge (Serbia, Yugoslavia). The testing carried out by the Republic Hydrometeorological Service of Serbia (Yugoslavia), has been in compliance with the program of testing the Sava river water and sludge quality along the river flow. The establishing of sludge pollution range is not based upon values of maximum allowable concentrations. The most complex is the issue of heavy metals contents, since these also represent the natural contents of soil and minerals. However, considering the process of migration and distribution of heavy metals, and other harmful and dangerous matters discharged from utilities, industrial and power supply facilities, provides the basis for estimating the environmental pollution as well.