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significance, as well as an upper part of Cretaceous rocks. This zonę does not reach a depth of 200 m (Różkowski, Rudzińska 1982).

Geological and geomorphological conditions effect the character of water circulation. The extensiveness of water and very moist areas, as well as a very shallow position of the groundwater-table favour a high evapo-transpiration. From the balance calculations madę by Michalczyk (Wilgat et al. 1991) it results that over 80% of precipitation water returns to the atmosphere. The remaining runoff consists of almost of eąual parts of overland (48%) and subsurface flow (52%). The eąuilibrium results from land flatness making water runoff difficult, and limited absorbance of ground which cannot storę a large excess of water, particularly in spring.

The runoff rhythm typical for rivers with rain-snow supply in this part of Europę distinguishes by the Polesie rivers from those flowing in the neighbouring regions. Astrongly marked maximum runoff in March is followed by a minimum in July unlike that of other Polish rivers in the summer-autumn months. This is a conseąuence of weak underground supply, the reserves of which are readily exhausted. Most water runs off in spring (III-V) — 41%, much less in summer (VI—VIII) — 18% and still less in autumn (IX— XI) — 16% when underground reserves are exhausted and the atmosphere supply is very smali. In winter, runoff increases to 25%, as a result of irregularly occurring thaw.

The specific discharge is smali, on average a little below 3.5 1/s km², and even below 0.5 1/s km² during long-standing droughts. The smali amount of discharged water does not favour the formation of a well-developed stream network. There are few rivers flowing in distinct valleys. Ali streams carry little water. The biggest, the Wlodawka river, discharges an average of over 2 m³/s of water into the Bug river. Nonę of the remaining rivers in the Lakę district carries 1 m³/s.

The rivers flow E into the Bug, and W and NW into the Wieprz. The watershed dividing the two catchments runs across the middle of the Lakę Region. Djstinct on Cretaceous hills, it disappears in fiat soaked depressions, where artifical ditches form water ’gates’ in it.

River sections near the watershed, which at present cannot be distinguished from artificial ditches, have no developed valleys and their channel slopes are smali. Farther from the watershed the channel slopes are bigger and the valleys morę distinct. This accounts for the immaturity of the hydrographic system; the rivers have not yet reached the central part of the region. Accordingly, a part of the lakes have not been included in the drainage network.

Lakes are the main element of the Lakę Region’s hydrosphere. Cataloguing performed at the beginning of the 50’s showed the existence of 68 lakes of over 1 ha in area (Wilgat 1954). They are smali bodies — the largest is 284 ha — of non-diversified shapes, often close to a circle or an oval. The lakę depths are differentiated; shallow and very shallow ones predominate. The deepest exceed 30 m, which with the smali surface areas gives big mean