Sediment balance of the Volga reservoirs IM. A. Ziminova Abstract. The main input component of the Volga Reservoir sediment balance is the product of bank and bed abrasion (60—80 per cent of the total input). The second component is the river suspended sediment discharge (20-40 per cent}. Only 1 -S per cent of the total sediment input is derived from plankton and the higher aquatics. The main component of the output is sedimentation (60-98 per cent). Suspended sediment discharge from reservoirs varies from 2 to 40 per cent of the total output. Résumé. La plus grande partie des entrés dans le bilan de sédiments des réservoirs de la Volga se compose des produits d'affouillement des beiges et du lit, il en résulte 60-80 pour cent du total des entrées. La deuxième place est occupée par l'écoulement des matières en suspension transportées par les rivières (20-40 pour cent). Le poids qui résulte de la production du phy to plankton et des plantes aquatiques supérieures constitue 1-5 pour cent de la rentrée générale. La principale partie des sorties est la sédimentation (60-98 pour cent). Les transports solides en suspension sortant des réservoirs constituent de 2 à 40 pour cent du total des sorties. Sediment balances of reservoirs are being compiled to estimate the silting of reservoirs and to forecast the tendency of this process. These balances allow one to determine the value of and to reveal the causes of quantitive changes in the sediment discharge under the conditions of flow regulation, and to consider possible changes in the composition of sediments. Sediment balances have been compiled for the Ivankovo, Uglich, Rybinsk, Gorky and Kujbyshev Reservoirs. All these reservoirs have volumes of 1.2 km3 (Ivankovo) to 58 km3 (Kuj[byshev). All the reservoirs are used for seasonal regulation except for the Rybinsk Reservoir which is used for yearly regulation. The watersheds of the Ivankovo, Uglich and Rybinsk Reservoirs are situated in the zone of annual river turbidity of no more than 25 g/m3, while the watershed of the Gorky Reservoir is in the zone of 25-50 g/m3 and that of the Kujbyshev of 50-1000 g/m3. The equation of the sediment balance has the same structure for all the reservoirs of the Volga Cascade. Its input consists of river sediment discharge, products of bank and bed abrasion and of plankton and higher aquatic vegetation; the output consists of sedimentation, sediment discharge through hydro technical structures and the content of suspended matter in the water mass at the end of the period studied. The balance elements which in the long-term period show no trend (river sediment discharge, sediment discharge through hydrotechnical structures, and plankton) have been obtained from normal annual values. Values of the other balance components have been determined as averages for the whole period studied. The suspended matter in the water mass at the end of the period was negligible compared with the other components. The method of calculation has been given in our previous papers (Ziminova and Kurdin, 1972a and b). Our results are shown in Table I. Two opposing processes take place in a reservoir when river water carrying terrigenous deposits flows into it: enrichment of the waters by new suspensions and the deposition of suspensions both brought in from the watershed and formed in the reservoir. The quantity of autochthonous suspended matter entering the water mass from various sources depends on differences in the hydrological and hydrobiological processes. Data in Table 1 show that river sediment discharge into reservoirs is equal Sediment balance of the Volga reservoirs 429 TABLE 1. The normal annual proportions of the components of the Volga cascade sediment balance, % input and output Output input Sediment •Production discharge River of plankton through Reservoir, sediment Processes and higher hydro technical period discharge of abrasion aquatics Sedimentation structures Ivankovo 1937-1968 29 66 5 71 29 Uglich Î940-1968 39 58 3 56 44 Rybinsk 1941-1965 18 80 2 95 5 Gorky 1957-1966 17 82 1 95 5 Kujbyshev 1956-1966 41 58 1 98 2 to only 20—40 per cent of the total input of the suspended matter and the autochthonous suspensions make up 60—80 per cent of the total input. The most important of these are the products of water abrasion. The important role of water abrasion in the sediment balance of large plain reservoirs is associated with the nature of these reservoirs as specific geographical objects. In the majority of natural lakes input of matter due to bank abrasion is slight in comparison with river sediment discharge. In the plain reservoirs which are as a rule not more than 20—30 years old and whose basins are far from being completely formed the bank abrasion is more intense than in iakes of the same geographical zone. The wave action of the water mass results in the transformation of banks and erosion of shoals. By treating bank transformation as the major factor in the silting of large plain reservoirs scientists ignored, as a rule, the second consequence of the abrasive action of a water body — the erosion of bottom areas above the level of wave action. Only now has the necessity to take this factor into account become apparent, This has led to attempts to estimate it. We have shown for the Rybinsk Reservoir that the quantity of autochthonous suspended matter formed owing to erosion of such areas is quite comparable with the quantity of matter formed owing to the bank abrasion and under certain combinations of morphometrical and geology- morphological conditions may surpass it. Input of suspended matter from this source is more intensive during the first period of reservoir existence — the period of heavy bank transformation and formation of shoal relief. With time the tendency to a lower rate of bank abrasion appears and a gradual decrease of suspended matter input owing to shoal erosion takes place as the formation of shoal relief is completed. The last signifies that dynamic equilibrium between mechanical composition of deposits and hydrodynamic activity of the water mass has been reached on the shoals. The duration of shoal relief formation depends on the type of stream flow regulation. In the Ivankovo Reservoir where we find the same annual storage and decrease of storage the duration of shoal formation was about 10 years. By that time part of the shoals was overgrown with higher aquatics, which indicated the completion of shoal relief formation. In the Rybinsk Reservoir this period was equal to 25—30 years. It is natural that the share of abrasion material in the annual balance of this period was more than its normal annual value. The production of the phytoplankton is the chief biotic factor enriching water in suspended matter. Its average annual value under 1 m2 surface is about 170—460 g of 430 N. A. Ziminova dry organic matter. Production of the higher aquatics is 6—27 times less than that of phytoplankton. The greater part of the production of the phytoplankton (80—90 per cent) and the higher aquatics (about 50 per cent) is mineralized in the water. The remaining suspended matter (1 -5 per cent of the total input) which is not easily mineralized is involved in the transport and sedimentation of suspensions. Such an estimation of the role of biotk factors in the sediment balance is arbitrary since transport and sedimentation take place during all stages of mineralization of new organic suspensions. The proportion of biotic suspensions discharged from the reservoir before reaching the indicated degree of mineralization is not accounted for in the input part of the balance. Taking into consideration the high rate of mineralization during the summer it is possible to suggest that this error is not significant. If the sedimentation of slightly mineralized organic suspensions takes place, their destruction continues on the bottom and the fixed bottom deposits include the products of this process. The greater part of suspensions discharged into a reservoir with river flow and formed in it accumulates on the reservoir bottom and forms bottom deposits. The size of these accumulations depends on the total input of suspended matter and hydrological—morphological features of reservoir. Morphometry mainly influences the total input of suspended matter on the unit of bottom surface. In all the reservoirs the values of the intensity of suspension formation owing to both zonal and azonal factors are of the same order. The Rybinsk Reservoir is the largest one in the Upper Volga. As a result the total input of the suspended matter to it (about 103 tons/km2 year) is half as high as that in the Ivankovo and Uglich Reservoirs (about 2 X Ï03 tons/km2 year). However, in the Rybinsk Reservoir sedimentation is equal to 95 percent of input suspensions and in the Ivankovo and Uglich it is 71 and 65 per cent respectively (Table 1). That is why the difference in the weight of deposits on the I km2 of bottom area is less significant (Table 2). The remarkable discrepancies between weight and TABLE 2. Sedimentation rate in the Upper Volga reservoirs Weight of the bottom deposits (103 tons) — - Volume of the Per bottom deposits Average Duration annum (kms) height of of rated on km2 deposit period Per Per of Per Per stratum Reservoirs (years) period annum bottom period annum (cm/year) Ivankovo 32 15,665 490 1.5 0.02120 0.00066 0.19 Uglich 29 8,030 277 1.1 0.01230 0.00042 0.17 Rybinsk 25 103,200 4128 0.9 0.283 0.0113 0.25 depth of the Rybinsk Reservoir's sedimentation is accounted for by features of its deposits.