resources of any river in the Polish Carpathians. The water of the Poprad degrades the water quality of the Dunajec. These two cases (the Oder and the Poprad) constitute the only problems along the southern section of the Polish border with which the divide coincides.

A long section of the eastern border conforms to the courses of the rivers San and Bug. There are no significant problems with inflows of pollution from Ukraine within the drainage basin of the San. In fact, the water of this river's upper course is of natural chemical composition, which is a rare phenomenon in Poland. In contrast, the Bug, whose middle section forms the border with Ukraine, is one of Poland's more polluted rivers. Human activity and unfavorable natural conditions both cause problems. The Bug has a continental type of hydrological regime in which the flow increases considerably as snow melts in March and April, followed by months of consistently low flows. Effluents from the Ukrainian coal and food processing industries contaminate the Bug. The most catastrophic situation arises in autumn, when the sugar beet campaign begins and large quantities of waste water from sugar beet factories pour into the river.

There are many post-glacial lakes in northeastern Poland. The rich water resources of this area are lightly polluted, and water flows via small rivers to the Neman River into Lithuania and Russia.

Most conflicts regarding rivers flowing along the Polish border or into Poland regard water pollution. The Oder and Nysa Luzycka do not reach any norm for water purity at any point along their entire lengths; the water is beyond classification. To rectify this problem, both Poland and the Czech Republic must comply with management norms for water and effluents, and this is not likely to happen in the foreseeable future. The most significant progress has been in the regulation of pollution in the Poprad, as construction of a sewage works in Slovakia has begun. However, more extensive water management in the Poprad's drainage basin has been deferred since it would primarily benefit Poland. Although the Ukrainian side of the Bug's drainage basin is poorly managed, contemporary socio-economic changes should limit or stop the exploitation of hard coal, which would obviously help eliminate the discharge of the most burdensome effluents. Particular attention should be paid to maintaining, if not improving, the current state of purity of the upper San River in the Bieszczady National Park. This will only be possible through strict compliance to an agreement with Ukraine.

Poland's changing relief, and, later, human activities led to its hydrographic shape. In the glacial period, great rivers with latitudinal courses took the meltwaters of the Scandinavian ice sheet westward to the North Sea. The retreat of the ice sheet and the emergence of a river network resulted in a natural connection between the drainage basins of the Vistula and the Dnieper. The flat, marshy area in Belarus and Ukraine, known as Polesie, acted as a bifurcation area in the spring. Some waters of Polesie headed for the Dnieper and subsequently for the Black Sea, while others reached the Vistula via the Bug's tributaries. This natural link only disappeared in the nineteenth century, when the drainage of the marshes and bogs of Polesie began. The land became drier, spring meltwaters flowed away