FRIEND: Flow Regimes from International Experimental and Network Data (Proceedings of the Braunschwei] Conference, October 1993). IAHS Publ. no. 221, 1994. Impact of large-scale amelioration on the hydrological regime of Belarusian Polessye rivers B. V. FASHCHEVSKY Central Research Institute of Complex Water Resources Use, Minsk 220086, Republic of Belarus Abstract Studies of man's impact (amelioration activities accompanied by the creation of reservoirs, straightening and diking of channels, intensified agricultural production, etc.) on qualitative and quantitative indices of the river regime have been performed. Specific quantitative changes in the characteristics of annual, maximum and minimum flows, in slopes and tortuosity, as well as the hydrochemical parameters (pH, BOD5, COD, 02, CI, S04, N02, HC03, phenols, oil products, surfac­ tants, heavy metal salts), over many years are described here. INTRODUCTION Large-scale amelioration activities such as the construction of open drains, dense drainage networks, lockage systems and reservoirs, river channel straightening and diking, as well the increased amounts of mineral fertilizers and pesticides applied on the newly-developed lands of Belarusian Polessye have caused critical changes in the hydrological (hydrochemical, thermal, etc.) regime of the rivers. A great number of papers e.g. Bulavko (1971), Drozd (1984), Shebeko (1983), Moklyak (1973), Karkutsiev (1982) and others are devoted to the problems of amelioration effects on the rivers of Polessye as a whole, including the areas both of Belarus and the Ukraine. For years, these problems have led to contradictions among scientists due to the directly opposite results of amelioration effects. The problem is that various natural conditions (marshlands, underlying soils, terrains, soil moisture, etc.) and even amelioration works (open drains and closed drainage, lockage, deepening and straightening of channel, etc.) have often been ignored. CHANGES OF THE WATER REGIME Several techniques have been applied to estimate the modifications to flow charac­ teristics under amelioration effects: - by relationships with the flow-forming factors (precipitation, snowpack, air and soil temperatures); - by relationships with the analogous river flow unaffected by amelioration activities; - by fitting probability curves of flow characteristics preceding and following the amelioration activities (provided long-term records are available). The coefficients of annual, minimum and maximum flow changes as well as those of flood-plain duration and height are given in Table 1. The data analysis shows that, in general, the annual flow values at the majority of rivers are increased, except River 214 B. V. Fashchevsky Table 1 Changes of annual, minimum and maximum flows characteristic of Belarusian Polessye rivers. River - point Catchment Coefficients discharge change in the years of different frequency: area (krr annual minimum maximum submersion of flood- mean plain 50% 50% 50% 5% duration height Yaselda — Beryoza 1040 1.30 1.30 0.90 0.92 0.68 0.82 Yaselda - Lenin 5110 1.40 1.80 1.40 1.50 1.29 1.34 Bonrik — Parohonsk 1450 1.10 1.60 0.92 0.90 - - Tsna — Dyatlovichy 1100 1.45 2.15 2.00 1.50 1.45 1.08 Goryn — Rechitsa 27000 1.35 1.60 - - - - Sluch — Novodvortsy 813 1.15 1.25 1.20 1.20 - - Sluch — Starobyn 1780 0.80 0.95 0.55 0.45 0.56 1.04 Ubort — Krasnoberezee 5260 1.20 1.25 - - - - Pytch — Luchytsy 8770 1.25 1.30 - - - - Oressa — Verhutyno 520 1.25 2.40 0.85 0.75 0.76 0.88 Oressa — Luban 1290 1.15 1.80 0.45 0.50 0.09 0.11 Vyt — Borysovshyna 782 1.35 3.20 1.30 1.20 - - Slovechna — Kuzmychy914 1.20 1.70 - - - - Cherten — Nekrashevka 420 1.20 1.70 - - - - Kopaywka — Chersk 382 1.35 2.10 - - - - Lan — Loktyshy 909 1.00 1.60 1.16 1.10 0.94 0.94 Pulva — Vysokoe 317 1.20 1.50 2.20 - - - Sluch, downstream of the Soligorsk Reservoir. Here, the extra evaporation losses and great amounts of water utilized at the integrated potassium plants and other industries result in flow reductions to below normal. The annual flow at River Lan, near the town of Loktyshy remains unchanged. Changes of spring flood maximum values preceding and following the amelioration activities are characterized both by their decrease (Yaselda-Beryoza, Bobrik-Harohonsk, Sluch-Starobyn, Ptych Luchytsy, Oressa-Luban, Oressa-Verhutyno) and increase (Yaselda-Senin, Tsna-Dyatlovychy, Sluch-Novod- vortsy, Vyt-Borisovhyna, Lan-Loktyshy, Pulva-Vysokoe). The spring flood maximum reduction can be explained by the enlarged depth of the aerated soil layer, channel straightening, changes in the river network density, reservoir maximum relief, etc. The Luban and Soligorsk Reservoirs are typical of situations where the floods downstream of the control section are reduced to half the natural maximum amplitude. It is interesting to note that on River Yaselda at the two control sections different amelioration effects on maximum flows are observed. Upstream of the town of Beroyza, drainage systems and Selets Reservoir have been built causing some reduction of the Impact of large-scale amelioration on the hydrological regime ofBelarusian Polessye rivers 215 maximum (by 10%). At the lower reaches, upstream of Senin Village, the maximum water flows are increased due to River Yaselda's main channel embankment and cutting off the flood-plain. In general, the hydrographical changes of the Polessye river network which influence the maximum's formation and changes, should be considered in more detail. In the Pripyat basin (including the areas of both Belarus and the Ukraine) the open canal network density is increased more than twofold - from 0.32 km km"2 to 0.68 km km"2. Taking into account the closed drainage, it amounts to 1.61 km km"2, being five times as large as natural drainage indices. The more striking examples of the canal network density changes are observed on small streams. On River Yaselda (near Senin) the network density is increased by a factor of 7.7, and taking account of the closed drainage by a factor of 26. The density of the open canal network at River Bobrik is increased by a factor of 5 and taking account of the closed drainage by a factor of 10. In general, the small streams of Belarusian Polessye are characterized by the open canal network density increase by a factor of 2-3, and taking account of the drainage network by a factor of 5-10. The river channel straightening leads to changes of tortuosity, length reduction and stream slopes increase. As seen from Table 2, the slopes essentially increase amounting to 20-60%. Likewise, the tortuosity correspondingly declines. The character of flood-plain inundation significantly changes during spring floods. On the rivers with declined maximums, the inundation duration and water height on the flood-plain decrease. This is evident on the rivers which have been regulated by reservoirs (River Sluch, near Starobyn and River Oressa, near Luban). It should be noted that near Starobyn, the inundation height remains unchanged, and even rises to some extent. Meanwhile the water stage duration on the flood-plain is halved. The flood-plain at River Oressa near Luban is no longer flooded (water height and duration decreased by 90%). As a result, the conditions are impaired for fish spawning and growth, meadow plant reproduction, etc. On the rivers with increased maximum the inundation duration and water height increase (rivers Yaselda, Bobrik), but at lesser degree than the maximums. On most of the rivers, the minimal daily winter discharges have increased by a half or doubled due to an increase in storage capacity, when water is accumulated in warm periods and used in winter. The minimal discharges have declined at Starobyn (to zero) resulting from the regulation of the Soligorsk Reservoir regime. Table 2 Changes of slopes and tortuosity at river reaches after regulation. River Length of Slope % Coefficient; Tortuosity: Coefficient straightened before after of change of changes reach (km) regulation regulation natural straightened channel channel Tsna 1.6-65.5 0.232 0.367 1.58 1.37 1.12 1.22 Bobrik 30-56 0.219 0.266 1.22 1.29 1.06 1.29 Lan 25-76 0.239 0.378 1.58 1.63 1.03 1.58 Moroch 13.5-27.5 0.264 0.336 1.27 1.31 1.026 1.28 216 B. V. Fashchevsky CHANGES OF THE HYDROCHEMICAL REGIME To estimate the anthropogenic effects on the hydrochemical regime of the Belarusian Polessye rivers, a comparative analysis is made of the natural (background) and current conditions with almost similar water availability at various phases of the water regime (spring flood, summer and winter low flows). The results of studies by Sivko (1956) are applied as background data obtained for the summer low-flow period of 1953. The results to be compared with are taken for the 1980 (1981, 1984, 1985, 1986, 1987) periods of similar water availability. Besides, data provided by the Belarusian Hydrometeorological Survey for the period preceding the intensive amelioration activities (1952-1962) and following the large-scale economic development of the Polessye Region (1981-1988) are considered. The comparative analysis indicates that pH remains unchanged in the summer low flow period independent of the intensive economic activities on most of the rivers concerned (rivers Pina, Bobrik, Lan, Sluch, Moroch, etc.). The dissolved oxygen content increased by 20-30 % as compared to that of the natural regime, owing to improved water aeration on the works and, sometimes, to complete disappearance of peatlands in the river basins. The contents of slightly oxidated organic matter abruptly increased. Thus, BOD5 on the River Pina, near Pinsk increased by a factor of 3.3, on the River Lan, near Mokrovo by a factor of 2.2, on the River Sluch by a factor of 2, on the River Oressa, near Andreyevka by a factor of 3, etc.