Stream network response to rapid base level changes: preliminary appraisal in the Urmia Lake catchment (northwestern Iran)

Over the last two decades water level has dramatically dropped (i.e. about 7 m in period 1995-2014) in the Urmia Lake (north-western Iran) (Sima and Tajrishy, 2013; Tourian et al., 2015; ULRP, 2015). Such rapid change of water level is likely having significant effects on the stream network draining into the lake. The aim of this work is to give an overview of the potential effects of rapid base level changes on a stream network, specifically channel adjustments (e.g. incision) and related environmental impacts. The work does not analyze in detail stream dynamics in the Urmia Lake catchment, but it could represent a starting point for future studies on channel adjustments and related impacts in this catchment.Field and laboratory studies have documented that rapid base level drop causes a sequence of channel adjustments which may vary in relation to a range of factors, such as channel composition (i.e. bed and bank material), original cross-section, and channel slope. Using the approach proposed by Lane (1955), channel incision can be predicted in response to base level drop. In fact, base level drop should produce increase of stream slope, excess of stream power, and, eventually, bed-level lowering. Referring to models that consider sequences of evolution stages triggered by external disturbances (e.g. Simon and Hupp, 1986; Bollati et al., 2014), widening (or narrowing) could be associated with channel incision and other processes, e.g. downstream sediment transfer and deposition, could be expected. Specifically, those models point out that an incised stream may go through sequences of change (i.e. stages of channel evolution) and sequences may also change along the unstable stream.Stream evolution and processes have been studied in some terminal basins undergoing level changes similar to those that are occurring in the Urmia Lake (Kroonenberg et al., 1997; Bowman et al., 2010; Skorko et al., 2012). For instance, Bowman et al. (2010) showed that the continuous and rapid base level drop of the Dead Sea (i.e. about 0.5 m/y in the period 1945-2008) caused notable vertical incision of the stream network, without significant evidences of other processes (e.g. channel widening) at the present stage of channel adjustment.According to widely accepted channel evolution models and to other case studies of rapid base level changes, a preliminary framework about stream channel evolution in the Urmia Lake catchment can be drawn. Since it is very likely that channel adjustments are taking place along the streams draining into the lake, it would be worth to address future studies in the Urmia catchment to the following issues: (i) magnitude and temporal trend of channel adjustments (e.g. incision, widening, etc.); (ii) spatial extent of adjustments, e.g. propagation from river mouth; (iii) other factors, besides base level drop, that may drive stream evolution (e.g. dams); (iv) effects of channel adjustments on environment (e.g. ecology, groundwater resources) and structures (e.g. bridges, roads).