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Anthropogenic changes of water conditions..

production have considerably reduced water consumption in the city. The amount of wastewater discharged into rivers has also decreased by about 50 million m³ in 1983 to 42 million m³ in 1992. In recentyears, all wastewaters have been treated mechanically and biologically, which has considerably improved the ąuality of the Bystrzyca river water in its lower course. The Chemical composition of water in the upper course of the rivers is approximate to the natural. In the city area, wastewater and water from rain canalization, which cause ąuality changes of top waters, are discharged into the rivers. The polluted Bystrzyca river is a substantial threat to the ąuality of underground water in the area of lowered water table.

TABLE 2. Mean unitary flows and outflows in the years 1982-1991 according to data from the Institute of Meteorology and Water Management, Warsaw Division, and the Department of Hydrography, Maria Curie-Sklodowska Uniyersity in Lublin

River basin	Water-gauge	A [km^2]	Q [m^3/s]	q [1/s km^2]
Bystrzyca	Sobianowice	1265	3.95	3.12
Bystrzyca	Lublin	748	1.67	2.23
Czerniejówka	Wólka Abram.	104	0.25	2.40
Ciemięga	Pliszczyn	152	0.49	3.22
Differential basin	of Lublin	261	0.03	0.11

A — area, Q — flow, q — unitary outflow.

CONCLUSIONS

Lublin has, sińce its establishment in 1317, influenced the water conditions of the Bystrzyca river significantly. The character of the influence and its rangę have changed over the centuries with the development of the city and economic utilization of waters.

The first period lasting from the 14th to 17th c. was characterized by exploiting the Bystrzyca river and its tributaries to obtain energy. In that time many dammed-up mili reservoirs of various sizes were built. The rivers lost their natural character and looked like a series of flux reservoirs. This affected rubble movement significantly, accumulation processes in the valleys and the state of top and underground waters. Water exploitation for communal and craft needs was limited and did not influence water balance.

The second period lasted from the end of the 18th to the middle of the 20th c. In that time the energetic significance of the rivers declined, a situation connected with the introduction of steam engines and then combustion and electric motors. The particular dammed-up ponds were successively liąuidated or their height was reduced. The ponds which were primarily of large economic importance to the city then became barriers to its development and a sanitary threat. Thus subseąuent liąuidation of the ponds, regulation of the rivers