DISCHARGE FLUCTUATIONS

A wide variation of abandoned alluvial meanders has already been discovered on the Great Hungarian Plain by many Hungarian geographers and geologists. The me­

ander characteristics measured for active rivers have also been determined on these abandoned meanders. The abandonment of the channels has been dated using

radiomet-Fig. 6. Spatial distribution o f the two river groups. - A = coarse bed sediment - greater bed load and gradient; B = fine bed sediment - lesser bed load and gradient

ric (14C) dating methods, pollen analysis and geomorphological considerations. From the data collected it has been possible to define the age -of the abandoned bend and the dis­

charge at the time of abandonment, for several rivers. The results provide quantitative evidence of the fluctuation of the discharge of paleochannels during the last 13,000 years of the Late Glacial and the Holocene, and permit an outline of the general pattern of the Great Plain’s paleohydrology. The changes of the Late Glacial-Holocene dis­

charges thus estimated are shown in Figure 8. On the basis of a new relationship (Figure 9) between runoff and two climatic elements (annual mean temperature and annual pre­

cipitation) the author attempted to determine annual mean precipitation from Holocene discharges estimated by morphometric methods, for some periods.

These data have geomorphological consequences to be discussed in the fol­

lowings.

Fig. 7. Relation between the water discharge and meander length after the two Hungarian river groups separated on the basis o f the meander characters. - 1 = group A; 2 = group B; 3 = regression line (A.

group: Y = 78,4 X 0,46 (R = 0,95); B. group; Y = 80,3 X 0,36 (R = 0,89), where Y equal with the mean­

der chord length, X with the mean discharge o f the river)

th years BP

Fig. 8. Holocene fluctuation o f the water discharge o f the Hungarian rivers

The river activity and climatic change during the Late Glacial and Holocene in the light o f the changing paleoenvironment and the discharge fluctuations o f the

Hungarian rivers a) Changes in the paleoenvironment

Geographical studies during in the past 30 years in Hungary supplied concrete evidence as regards the environmental changes of the Late Glacial-Holocene period and a wide range of indirect data for a new synthesis on the chronological order of river activity (crosional, accumulational and transitional phases) in Hungary. The collected data are illustrated in Figure 10.

m ean annual temperature (°C)

Fig. 9. Relation between the climate and the runoff on the water catchment area o f the Zagyva river (Hungary) after Nováki, B. 1991

The most detailed information has been gained from palynology for the annual, July and January mean temperature changes in the Great Plain (7, 2 and 3 rows).

Relevant data have been obtained from cave sediments in the North Hungarian Mountains, where remnants of small mammals (voles) have been examined (Ja). A curve of the humidity change (so-called Arvicola humidity) is drawn (4) on the basis of this research.

A more recent method is based on the terrestrial Mollusc fossils to determine the July mean temperatures (lb) during the Upper Pleistocene and Holocene.

Additional information for environmental changes could be obtained from wa­

ter level oscillations of Lake Balaton (5/a-b).

Geomorphological research has established several stages of eolian sand movement. The new radiometric data on periods of soil formation on the dunes (ps. of humid conditions) indirectly determine the deflation periods (7). But the results did not fit into the old theories. Consequently, this also called for a reconsideration of the evo­

lution of the drainage system.

The stratigraphic evaluation of archaeological excavations also enriched our knowledge of the environmental changes in the Holocene period. Especially archaeo­

logical findings supplied evidence for the periods of alkalic soil formation, or those under arid conditions (8).

b) The mechanisms o f the Hungarian rivers in the Iuite Glacial and Holocene

For the assessment of the rate of fluvial erosion or accumulation, one of the in­

dicators is the water discharge. The discharge of the Great Plain rivers during the Holo­

cene based on morphometric methods, have been re-evaluated according to the new chronological order of the paleoenvironmental changes (Figure 10., row 6). The second indicator consists of the changes in sediment load, but in this respect only uncertain estimations are available.

In the light of these calculations and of the more recent geomorphological and sedimentological evidence, the changes of the erosional, accumulational and transitional phases of the Hungarian rivers and their chronology were established. The dotted areas mark the accumulation (upfilling), the single-hatched ones show the transitional (valley widening) phases, and the cross-hatched ones indicate the erosional (downcutting) phases (row 9).

c) Geomorphological consequences

The last Pleistocene river terrace (denoted in the Hungarian literature as Ha) was previously dated Holocene because of the absence of loess cover on its surface and the presence of an eolian sand cover (Pécsi, M. 1959). This sand accumulation was dated earlier Boreal, but recent investigations (Borsy, Z. et al. 1985, Lóki, J. et al. 1995) could only find evidence for eolian deposition during the Pleistocene, especially in the Pleniglacial and Late Glacial (Early and Younger Dryas). According to the latest

evi-during the Pleistocene as a result of the Alleröd erosion. During the Younger Dryas valley accumulation and occasional lateral erosion was characteristic. Sand dunes were formed not only on the flood-free terrace, but everywhere on the marginal alluvial fan surfaces too.

At the beginning of the Preboreal the increased water flow and the reduced sediment load caused a downcutting phase for a short time. Accumulation started during the second half of the Preboreal and in the first half of the Boreal (accumulation of the material of the Early Holocene or terrace I). The next humid period occurred during the transition between the Boreal and Atlantic phases. The palynological examinations show considerable differences in the climate of this period within the Carpathian Basin: the eastern part of the Great Plain was much more continental. The more frequent floods and extreme water discharges of this wet phase resulted the next phase of valley deep­

ening (cutting into the Early Holocene terrace) in the Boreal to Atlantic transition.

The low stage of Lake Balaton and pollen analyses have proven that at least the second half of the Atlantic phase was much drier then hitherto was thought of. The pa- leohydrologic reconstruction by radiocarbon dating shows that some Atlantic (6200 B.P.) channels of the Tisza river (Borsy, Z. and Félegyházi, E. 1983) had lower water discharge than today (Gábris, Gy. 1985, 1986). The stratigraphical evaluation of ar­

cheological excavations indicates dry climate too (Bácskai, E. 1991). Alkalic soils hav­

ing formed above the middle Neolithic cultural layer point to an excessively continental climate, with negative water balance. There are similar alkalic soils in some middle Copper age (approx. 4500 B.P.) settlements too. Due to the dry climate and low dis­

charges the rivers were silting up during the second half of the Atlantic and the first part of the Subboreal phases.

The palynological and zoological data indicate that the latest intensive down­

cutting phase occurred probably during the second half of the Subboreal. Due to the

gary. 1. July average temperature; a. "Vole-thermometer"; b. "Malaco-thermometer"; 2. Mean annual temperature; 3. January average temperature; 4. "Arvicola humidity"; 5. Water stage changes o f Balaton (a - Western basin, b - Eastern basin); 6. Variation o f the river discharges; 7. Deflation periods; c. Wind blown phases; d. Period of soil formation on the dunes; 8. Periods o f alkalic soil formation; 9. Type of river mechanism (dotted areas - accumulation, single-hatched - transitional phase, cross-hatched - down­

cutting).