TERRACES AND FLOODPLAIN LEVELS

According to the above considerations on the surface processes four major geomorphological levels were formed in the Great Hungarian Plain. Two of them are of Pleistocene origin whereas two are of Holocene age:

On the margin of the Great Plain many flat alluvial fans reach towards the cen­

tre of the basin in continuation of the mountain river valleys.

- The surface of these alluvial fans is partially covered by young (Late Pleisto­

cene) loess, or loess like sediments. This level lies 2-6 m higher than the Holocene flat inundation levels being an extensive flatland without any kind of microrelief. The thick­

ness of the loess sediment is only a few metres; thicker sequences can be found only in the peripheries of the plain and in the region northeast of Debrecen.

- Large areas are also covered by Pleniglacial eolian sands (having been de­

posited in two phases) and these formed in the Late Glacial (Dryas). The Holocene de­

flation of the sand areas in the eastern and the northeastern part of the Great Plain is not yet proved, but it is very probable in the Danube-Tisza Interfluve. The last and weak eolian phase is attributed to the changing land use in the XVIII century after the Turkish occupation. This morphologically diversified sandy level rises a few ten meters above the recent flood plain. Many kind of dune forms can be found here and in the small depressions evaporites are present too.

The Pleistocene surfaces are riddled by abandoned river channels of Late Gla­

cial and Early Holocene ages. This ancient rivers cut narrow but deep channels of dif­

ferent size. Most of the branches now are filled up and dry.

The Holocene levels are flooded areas. Before the river regulations of the last century more than one third of the Great Plain (about 2 million hectares) was perma­

nently or intermittently inundated by the floods of the Tisza and its tributaries. The dou­

ble fluvial cycle resulted in two Holocene levels:

- an extensive Early Holocene high flood plain surface was formed by the Middle Holocene downcutting phase of rivers and

- the areas of the currently still subsiding offspring-depressions and the current flood plains of the rivers belong to the lowest and youngest Late Holocene areas. These flood plains have variable micromorphology due to the abandoned channels and the recently sinking parts used to be marshy depressions with peat bogs in their natural condition.

CONCLUSIONS

The Late- and Post Glacial development of the drainage network in the Great Hungarian Plain was controlled predominantly by the uneven subsidence of the basin.

The young "offspring-depressions" along the margin of the Great Plain forced the Tisza to move from the axis of the basin to its current position during the Late Glacial. The recent studies revealed some changes of the river courses of the Tisza tributaries during the Holocene. The climatic variations affected the river discharge and the river mecha­

nisms. The paleohydrology of the Great Plain can be established using the equations between the river discharge and meander properties, calculated by the author. The chronological order of the river activity (downcutting, upfilling and transition phases) is established by an analysis of the environmental changes of the Late Glacial and Holo­

cene. During this time four major geomorphological levels - those of loess or loess like sediments, sand covered alluvial fans, high and low flood plains - were formed in the Great Hungarian Plain.

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