ON THE PROBLEM OF THE INFLUENCE OF KARST ON THE HYDROLOGICAL REGIME OF RIVERS A.M. GAVRILOV (USSR) Karst is widely spread in the European part of the USSR as well as in Eastern Siberia beginning from Krasnoyarsk. In the European territory of the Soviet Union the proportion of karstic fissured rocks-consisting mainly of limestone and gypsum- is especially great in the following “Karstic” regions: the Urals, the Belomorsko- Kuloiskaye plateau, the Onega-Dvinawater-divide area, the lake district to the south of Onega lake, the Silurian plateau to the south-west of Leningrad,the middle reaches of the Volga and some other regions. In the regions mentioned above covered karst is found as a rule. Sometimes a complicated alternation in the bedding of karstic and non karstic rocks can be observed which stipulates fairly diverse source conditions of some rivers-for instance in the Urals and in the middle reaches of the Volga. In other cases karst is represented by large massifs made up of homogeneous fissured rocks-for instance in the Silurian limestone plateau and to a considerable extent in the region of the Emtsa river basin in the Onega-Dvina water-divide area. Karst influences the sources and the hydrological regime of rivers and according to observational data in the northern and middle section of the European part of the USSR, this effect is important in rivers with drainage areas of not more than 3,000-7,000km2. Occasionally the influence of karstic sources on the hydrological regime is noticeable in some stretches of larger rivers. As is known,the influence of karst on run-offarises from the more or less complete absorption of precipitation by karst rocks and the existence of rather large groundwater storage. Consequently the share of the surface run-off decreases and in many cases the annual distribution of run-off is considerably regulated, with more moderate values of maximum discharges and higher values of minimum discharges. The lack of coincidence of the surface and underground drainage basins inherent to karstic regions often causes noticeable deviations of average rates of discharges of karstic rivers from the zonal normal amounts and these deviations may be of opposite signs even for neighbouring rivers. The long-rangeamplitude of annual run-offis smoothed down too. In particular cases the influence of karst upon the run-off greatly depends on the complex of local geological and hydrographical conditions, such as the character of bedding and the structure of karsting and underlying impermeable rocks, as weil as the availability of impermeable lenses in the bulk of fissured rocks and their associated “perched” storage of karstic waters. The direction of rivers also may promote or hamper the interception of karstic waters. Streamflow investigations on karstic rivers and the systematization of their observation data are of great importance to hydrologists concerned with hydraulic structures and mining operations. It js possible to obtain the run-off data of karstic rivers mainly by two methods: 1. By organizing special hydrological and hydrogeological field investigations of karstic and non-karstic rivers of a definite river basin; 2. By the use of run-off observation data on the hydrological network in karstic regions taking into consideration local geological and hydrographical conditions. 544 The first method is preferable as it allows one to undérstand more completely the complicated multi-factor relationships between run-off and natural conditions and to separate and evaluate the proper influence of karst on run-off. Now a broad development of detailed field investigations may not be possible for a number of reasons, but it does not diminish the importance of such work both for local practice and for general methodology. The practical and scientific methods of detailed investigations checked by experiments can be applied later to similar complex cases thus giving economy of time and expenditure. The character of karstic influence of karst upon run-off may also be derived from streamflow data of a drainage basin or region, published in water yearbooks. The completeness of such data depends on the density and representativeness of the network of stream-gaugingstations. In this report, examples are given to characterise karstic influence upon the runoff of small rivers within the following regions of the northern and middle zones of the European part of the USSR: 1. Silurian plateau; 2. Belomorsko-Kuloiskoyeplateau; 3. Onega-Severnaya Dvina water divide area; 4. Right bank region of the middle Volga. In the first of these regions, special investigations of runoff in the Oredezh basin were made; for other regions, the data of permanent stream gauging stations published in water yearbooks were used. In all cases the ratio of the average low water (summer) discharge and average winter discharge (Qy) for a given period of years was accepted as one of the criteria of the annual flow regulation; the periods of observation are equal for all stations to be compared. The average low-water summer discharge (Qs) is calculated as an average value from discharges for two summer months, that usually have the least streamflow rate in a given region, and the average winter discharge (Qu?)is calculated correspondingly for two winter months with the least stream flow. The choice of months is defined by the general character of the hydrological regime of rivers in a given region. To evaluate the natural yearly regulation of the runoff the following classes are defined: I. High regulation Qs/Qy N Qu;/Qy = 60%. II. Moderate regulation Qs/Qy - Qw/Qy = 40-60%. III. Slight regulation Qs/Qy -Qw/Q.v = 25-40%. IV. Negligible regulation Qs/Qy = Qw/Qy = 1-25;,;. The data obtained should be plotted upon schematic maps of summer and winter run-off regulation which given valuable descriptive characteristics. 1. KARSTAND RUN-OFF IN THE OREDEZHRIVER BASIN Hydrological investigations in the basin of the Oredezh river, tributary of the Luga river (Leningrad district) were organized by the State Hydrological Institute (SHI)during 1952-1954;one of their purposes was to establish the karstic influence upon the hydrological regime of rivers. The investigations were carried out according to the method of hydrological comparison,by means of systematic uniform observations of water discharge in the correspondingly situated gauging stations on karstic and non-karstic rivers of the basin being studied. The source of the Oredezh river is situated at the edge of the Silurian plateau, which consists of karst limestones; the river in its upper course flows to the East and 545 then turns to the South (fig. I). Observations at Vyritsa (drainage area A = 734 kma) and Morovino (A = 2890 km2), have shown that the greatest influence of karst upon run-off is shown in the upper course of the river from its origin up to Vyritsa. Seven new stream-gaugingstations were opened here and together with Vyritsa their number Fig. 1 -The scheme of disposition of stream gauging stations on the Oredezh and Luga rivers: 1. Oredezh r. at Zarechye; 2. Konevka r. at Zarechye; 3. Chernaya r. at Kordon; 4.Oredezh r. at Daimishtche;5. Oredezh r. at Siverskaya; 6.Oredezh r. at Belogorka; 7. Orlinka r. at Orlinka; 8. Oredezh r. at Vyritsa; 9. Oredezh r. at Morovino. 10. Luga r. at Voronino. reached 16 stations on the Oredezh river and 2 stations on its tributaries-the Chernaya and the Orlinka rivers. For comparison, data of the Oredezh river at Morovino (with negligible karstic sources) as well as of the Luga river at Voronino (no karstic sources at all) were also taken (table 1). TABLE 1 ~~ Drainage Percentage Percentage No. River Station area (A) of forest of swamp in km2 area area Oredezh B. Sarechye 113 40 5 Chernaya Kordon 29 80 40 Oredezh Daimishtche 230 55 12 Oredezh Sivers kaya 351 53 10 Oredezh Belogorka 390 53 9 Orlinka Orlinka 183 38 10 Oredezh Vyritsa 734 56 8 Oredezh Morovino 2,890 40 20 Luga Voronino 839 30 35 546 Values of the long-term average rate of run-off for rivers without karstic sources reach 7.0-7.5l/sec/km2 for this region. In the upper course of the Oredezh,the average rate of run-offaccording to ten years’observations is 1.5 to 2 times more than the zonal norm (table 2). It can be mainly attributed to the additional inflow karst waters of the Silurian plateau; besides, the underground drainage area exceeded the surface drainage area by 35% at Daimishtche and by 30% at Vyritsa. The amount of precipitation is 20% less in the upper course of the Luga river than in the upper course of the Oredezh river and this difference is relevant also. The influence of karst waters upon river discharges is most significant at the Daimishtche gauging station; further downstream this influence gradually decreases. The flow of the Orlinka and (especially) the Chernaya rivers approaches the zonal value since karstic waters only feed them in a minor way. TABLE 2 The average rate of run-offfor the period 1954-1963(l) Drainage area Average rates of River Station (A) in km2 run-offin I/sec/km2 Oredezh Zarechye 113 11.2 Oredezh Daimishtche 230 15.2 Oredezh Belogorka 390 13.6 Oredezh Vyritsa 734 10.4 Oredezh Morovino 2,890 1.4 Chernaya Kordon 29 8.0 Orlinka Orlinka 183 9.3 Luga Voronino 839 7.0 On the river reach considered the annual run-offof the Oredezh is largely regulated by the storage of karst waters, accumulated in epring (every year) and in autumn (50% of occasions). The most permanent regulation is observed near Daimishtche where there is no appreciable exhaustion of the source whose minimum rate of flow in the driest years does not become lower than 4.0 l/sec/km2.On non-karstic rivers the corresponding values may fall in the range 0.2 to 0.4 I/sec/km2.