Erosional chaos and unpredicted evolutions in Ardabil Plain

Rivers as natural channels for collecting and transport of water, has always been an important consideration for the location of human habitations. River capture is one of the most important changes in river system that may lead to changes in other parts of the system. River capturingis a geomorphological phenomenon occurring when a stream or river drainagesystem is diverted from its own bed, and flows in the bed of a neighboring stream. As hydrological systems are inherently nonlinear(Amorocho and Brandstetter, 1971), therefore using chaos theory despite its complexities seems to be one of the simplest ways to define and predict of behaviors of these systems. The concept of chaos implies a kind of regulation in the framework of an irregular trend. One of the important issues that has been emphasized in chaos theory is the occurrence of large scale phenomena as the result of small changes in dependent variables, called butterfly effect. Current study tries to evaluate the probability of occurrence of river capture in eastern part of Ardabil plain and also its future effect on the region has been discussed. This phenomenon is occurring in a basin with area of 74.31 square kilometers near eastern border of Ardabil plain.Despite the smallarea of the basin in comparison with the Ardabil plain (4621 km2), the changes in this basin could lead to changes in whole hydrological system of the plain. The study area in this research is Ardabil plain. Rivers flowing in Ardabil plain includeBalkhlichay and Ghorichayare joined in GharaSouriver. Total drainage area of the plain is 4621.48 km2.

In this study, topographic maps (1:50000), geological maps (1:100000), Digital elevation Models (DEM) and aerial photographs besides field observations were employedfor doing precise evaluations. In order to investigate and predict of river capture phenomenon, at first effective factors on erosion in twonear basins (Ardabil and near basin in eastern slopes of Talesh) were studied and then the most critical part for occurrence of river capturingwas definedusing distance measured between river branches of two basins. As geological baseof two basins are similar, therefore the rate of erosion in 2 basins must be the same, but field observations show that the erosional rate in near basin is faster than eastern part of Ardabil plain. Slope gradient could be responsible for this process. Ardabil plain has a slope of 2-28%. Eastern parts of the plainhave an average slope of 2-5%, middle parts 0-1% and in western parts up to 28% slope could be observed. In eastern part, in comparison with near basin (20%) the slope gradient is very low. Therefore slope gradient could be an important factor in capturing river in this region. In order to investigate the probability of river capturing, 35 profileswereplotted between two drainage networks branches in 2 sides of the slope, then the distance between them were measured usingArcGIS. The results show that in profiles no. 1-12 in eastern part of the basin, the low sloperesulted in high probability for river capturing. Profile 21 has the lowest distance (115 m) and is the most critical location for capturing in future. Profile no. 26,with distance of 185 m, is another susceptiblelocation for capturing. Headward erosion and river capturing will cause a large amount of input water in Ardabil plain exit from the basin. With headward erosion and capturing, rivers in Ardabil plain will find their ways to Armenia or Caspian Sea and both of these probabilities are potential threats for Ardabil plain. Also this phenomenon may have several impacts on agricultural activities and economy of the region and transportation network (Astara-Ardabil road) and especially Ardabil airport that is located near the critical point. Furthermore, humanactivities that increase the landslide may facilitate the headward erosion and could increase river capturing process.

Evaluation of erosional chaos in Ardabil plain implies that headward erosion in eastern part of the plain will have several impacts on the regionand with increasingheadward erosion, the occurrence of the capturing in eastern slope will be inevitable and hydrological system and drainage network will be changed totally and many constructions and roads and also Ardabil airport, will be in danger. To avoid raising this problem in future, more considerationsand planning need to be established and controlling activities in critical points is necessary.