The modern strategies for management of water resources point to the role of self-purification processes as a natural highly effective mechanism for maintaining of the ecological state in river ecosystems. The leading factor for the realization of self -purification processes in rivers is the functioning of microbial segment of biocenosis. The microorganisms occupy the nodal position in the fundamental transformation processes of organics and nutrients, as a result of their flexible metabolism and enormous physiological diversity. The functional profile of microbial community as a presence and proportion, relationships and dominancy of the key groups determine significantly the opportunities of ecosystem for adequate response to pollution with different origin. The state of microbiocenoses is a direct indicator for the effectiveness of the ecosystem functioning and its precise analysis allows a fine regulation of the entire self-purification capacity and water quality by specific controlling approaches and measures (RHEINHEIMER, 1992; SIGEE, 2005). The discussion of microbial biodiversity should include another important element - a sanitary bacteriological aspect of the problem, which has the most critical manifestation in strategic sources of drinking water. The provision of microbiologically pure water is a priority of any developed country. CHAPMAN, 1992, indicates the presence of pathogenic microorganisms as the most famous risk factor associated with water. The biodiversity and functional stability of autochthonic microbial communities are essential for the incorporation of potentially dangerous alochthonic microorganisms. The connection of microbial communities with water quality has a dual importance: the presence of certain families and strains directly endangering the human health, while the functioning of microbial communities in the entire spectrum of their physiological diversity is the main factor for the realization of self-purification processes and improve water quality. The aim of this paper is to provide useful information for microbial biodiversity in upper subcatchment of Iskar River and to specify the critical control points (as a time, location and parameter) for formation of microbiological factors of water quality. The functional contribution of microbial community for river metabolism and water quality has recently attracted more attention with respect to conservation, restoration, and management of water sources.

There are 3 major factors influencing the self purification process in river:1. Biochemical processes lead to the degradation and decomposition of organic wastes by organisms (including microbes and plants) in the water.2. Hydrochemical processes like oxidation, mineralization etc. may purify the water.3. Physical processes like adsorption, dilution etc. also aid in the same.Well, there is a limit to the natural healing process. Not all manmade pollutants (e.g. plastics) are degradable. Also natural healing process is a function that is time dependent. Therefore if the rate of

introduction of pollutants into the ecosystem is faster than the rate of its natural decomposition, the toxicants get piled up and result is detrimental.

the self purification process is sometimes delayed and and some times fastened in different seasons

the factors like water temperature - leads to change in oxygen exchange rate, amount of precipitation, intensity of precipitation in an area, characteristics of the precipitation like pH, physical form - fog, rain, hailstones also impact water temperatures, change in water contents like increase in silt brought by runoff decrease the sunlight infiltration thus delay in process, sunlight penetration also helps bacteria for fastening the decomposition, increase in nutrient content brought by runoff also increases the algae etc thus decrease in oxygen and delay or stagnation of self purification etc.. These won't happen in isolation - the velocity of water in a channel, the depth of water, the turbulence created by the particles are carried from upstream or eroded loose rock at the bottom, the change in velocity w.r.t width and the curvature of the stream, river basin topography etc and human impact factors like the effluent load per unit length and comparison of load with above all factors.

other factors affecting the self-purification process in a river include1. the geochemical profile of the substratum, ion exchange properties that determine mobilization or retention of metal pollutants for instance.2. the existing pollutant load of the sediment before another pollution event can determine the self purification process3. the incidence of other sources of pollution or anthropogenic impact down stream will actually will actually determine the reduced or increased recovery ability and recovery process of the river.4. the depth of the river and the steepness of the gradient along which it flows will determine the influence of oxygen in oxidizing volatile pollutants which is one of the recovery mechanisms or river. Fast flowing rivers due to steep gradients will have more interactions with atmospheric oxygen.

The self purification of natural water systems is a complex process that often involved physical, chemical, and biological processes working simultaneously. The factors that affect self purification would be dilution, wherein a sufficient amount of water can suffice and be available in a receiving amount of water. The dissolved oxygen level will be available since there is a sufficient amount of water for dilution. Also, temperature and sunlight can be factors that affect self purification in rivers since both acts and correlates with each other very well. It is a given fact that nutrients including dissolved oxygen are more present in cold temperatures. Self purification can occur more in cold temperatures rather than hot temperatures. For sunlight on the other hand, more sunlight produces more photosynthesis by algae. Sunlight helps add oxygen in the stream, thus leading to self purification.