Sediment Sampling
Twenty-five representative sediment samples from seven main western rivers of Urmia Lake basin were collected for this study (Figure 1). These included 9 sites in Nazloo Chai (S10 and S11), 3 sites in Shahar Chai (S13 to S15), 4 sites in Baranduz Chai (S12 to S15), 3 sites in Ghadar Chai (S16 to S18), 2 sites in Mahabad Chai (S19 and S20), 3 sites in Simineh Chai (S21 to S23) and 2 sites in Zola Chai (S24 and S25). Each river was sampled near the headwater and at the discharge point to the Urmia Lake Basin, and sometimes in-between. A total of two or three vertical sediment cores (0-10 cm) was collected from eah sampling site and then mixed to have a composite sample. River sediments commonly dry out as water level recedes and this process can be expected to influence P speciation (Schönbrunner et al., 2012). For the Urmia Lake basin, water levels rise and overland flow occurs seasonally in the riverbed sediments, therefore most samples were collected from dry sites.