Reconstruction of the Paleohydrology in Desert Areas Using Sar and Digital Topography Data

Arid zones have been affected during the quaternary by a large hydrological and climate variability. This change of processes has shaped the landscape leaving the signature of former paleodrainage and sedimentological features. Using the radar SAR images matched with the detailed topography, obtained from the SRTM (Shuttle Radar Topography Mission) data and ERS interferometry, it is possible to reconstruct the paleodrainages and identify the paleohydrological behavior of the past rivers and standing bodies of water. Interferometric data may show morphologies and provide elevation to estimate the former paleowater budget. Also a direct comparison between the past and previous settings is possible by the matching of SAR and other imaging product. We will present some examples from the Western Sahara (Algeria and Tunisia). 1. SEDIMENTOLOGY OF THE DESERT RIVERS The sedimentology of the rivers flowing in arid and semi-arid environments does not have attracted much attention in the past [1, 2]. Basically the sole model of “desert” rivers is the ephemeral one. Actually, the rivers in this climatic setting display a remarkable complexity, and show strong variability in facies and channel patterns [2]. In term of complexity they can be compared even with the pluvial-tropical rivers [3, 4]. Among the large variety of river systems we can recognize the following basic types: (i) the ephemeral streams which undergo episodic flooding and long inactive dry periods; and (ii) the exotic rivers, which are perennial even if affected by large discharge fluctuations. The ephemeral streams are usually short and with relatively small drainage basins. The major characteristic of the ephemeral streams is that they are affected by rare floods with high peak discharges. Currents are usually in the upper flow regimes due to both the high velocity and the shallow depth. Eolian sands accumulate in the river beds in between floods. These rivers occur in arid areas, even in the innermost part of the Sahara. Evaporation and infiltration play a major role in decreasing the river discharge by producing a remarkable loss of water. Consequently, the rivers lose capacity of transport and the channel become less defined, shallow and broad in cross section up to form, in the downstream end, a distributaries pattern of channels. The channels fade out spreading their waters over large unchanneled areas (mud flat). These distributary systems have been named terminal fan [5, 6] and pass downstream in sebkha environments. Exotic perennial rivers [7] flow from rainy areas across arid zones: typical examples are the Nile, Niger and Senegal rivers. Their common characteristic is to lose water along their courses without vanishing entirely. When these rivers are present in areic basins they reach the sea and form marine deltas. If they occur in endoreic basin they debouch in lakes. Along their courses, exotic rivers Fig. 1. Characteristics of exotic and ephemeral river channels. undergo hydrological crisis that produce tremendous loss of water. The decrease in discharge is matched by a decrease in capacity of transport and erosional power: channels become shallower and form an intricate anastomosed patterns. In these zones the floodplain extensively consists of swamps and marshes forming the so-called inland delta. Very well known examples of inland deltas occur along the Niger River and the Nile. In these cases the rivers survive the hydrological crisis (the Niger River looses 70% of its water), but in other cases the rivers fade out into the delta (see, for example, the Okavango River in Botswana). The inland _____________________________________________________ Proc. of the 2004 Envisat & ERS Symposium, Salzburg, Austria 6-10 September 2004 (ESA SP-572, April 2005) deltas share some features with the terminal fan, because both are distributary systems due to loss of water. The distribution of the flowing water in several channels is the direct product of the decrease in erosional capability and in the increase of accumulation of sediment. However, the exotic rivers form anastomosed patterns because they are able to built around the channels fine-grained floodplain deposits. Instead the distributary channels in the terminal fans rest simply at the top of pre-existing surfaces. 1.1 Arid rivers and climatic changes The former types of rivers are subject to the change in climate and environmental conditions. The exotic rivers are linked to source areas with long strong rainy seasons and occur in both deserts and savannas. During the pluvial periods the Sahara desert has been crossed by several of these type of rivers as it is testified by the large number of paleovalleys preserved in the highlands; whereas lacustrine deposits had been observed extensively in the lowlands, which are currently covered by erg or sand sheets. The perennial and exotic rivers during the pluvial periods affected both areic and endoreic basins. Mediterranean and Atlantic rivers of the northern segment of the coastline were short-headed, whereas the rivers flowing in two the Atlantic south of the Senegal River were long and with high discharge. Those that flew in the present day Sahara were mostly connected with endoreic basins. Fig. 2. River type modifications according to climatic changes.