Spatio-Temporal Variations of Discharge and Sediment in Rivers Flowing into the Anzali Lagoon

In the last few years, trend identification has become an important issue in hydrological time-series analyses; it is also a difficult task, due to the variety of models and the impact of climate change on the river flow regime. Due to the vital importance of the Anzali Lagoon to the environment of the region, and the threat to its health caused by the volume or amount of inlet sediments, we decided to study the changes in flow and sediment in the rivers flowing into the Anzali Lagoon. For the present study, the long-term monthly, seasonal, and annual sediment and discharge data of seven stations were obtained during the period 1985–2019. According to the available information, the trend of sediment load variation was investigated at different time scales. In this study, the Mann–Kendall statistical test, the double-mass curve, and performance fitting were used to assess the seasonal and annual trends in sediment and river flow. The results showed that at Aghamahale station, the low relationship between discharge and sediment compared with that at other stations was due to the low slope and constant water of the Behmbar River, which caused the sediments to settle and decreased their carrying rate. Moreover, Nokhaleh station had the largest share of sediment transfer to the lagoon during 2002–2012. Sediment details also show that the highest amount of sediment in all stations occurred in non-crop seasons—i.e., from October to January—and was directly dependent on the amount of rainfall in these areas. The results of the sediment analysis also indicate that the discharge and the subsequent sediment loads from upstream to downstream were high over the summer. Furthermore, the rivers downstream demonstrated springtime peaks in the sediment loads and discharge, probably owing to snow melting.