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The results of estimation of the probable annual fl ood fl ows with the use of various sets of data from a small agricultural lowland river are presented. The traditional statistical series are formed from the annual maximum (AM) fl ows of hydrological years. After examination of the homogeneity of the series, two sets of data were formed (AM1 of 34 elements and AM2 of 40 elements), which differed in the signifi cance level of one of the stationary tests. The other series for fl ood frequency analysis has been formed by selecting peaks over threshold discharges (POT). A computer program, developed by IMGW (Institute of Hydrology and Water Management), and spreadsheet were applied for frequency analysis with the use of AM series and POT series, espectively. Results of computations with the use of AM1 and AM2 data indicate for nonsignifi cant differences in probable fl ood fl ows, and signifi cant with the probable fl ood fl ow estimated with the POT data.
Wetlands in the Middle Biebrza Basin have been transformed as a result of hydrotechnical schemes carried out in the area starting from early in the 19th century. First changes were triggered by the construction in 1824-1839 of the navigable Augustów Canal and were signalled in areas along the canal by the condition of the plant cover and soils. Major transformation followed the digging of drainage canals as part of public works organized in the wake of the year of excessive rainfall in 1844 to provide the starving population with a subsistence means. As a result of this project carried out over the 1845-1862 period several large canals were constructed, including two separating and straightening the Elk (Rudzki Canal - 17 km) and the Jegrznia rivers (Woźnawiejski Canal - 9 km). Disruption of the hydrographic network led to a partial draining of wetlands in about half of the area of the Basin. The first specifically agricultural drainage projects designed to bring tracts of peatland under management were implemented in the Kuwasy Fen (c. 660 ha) in 1933-1939. Work resumed on a more extensive scale in 1952 resulted, until the present moment, in the draining and bringing under cultivation of about 18000 hectares. The effects of the draining action of canals, particularly those dating back to 1845-1862, have to be considered when planning protection of areas lying in the Biebrza National Park.
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