Undesirable effects of sand and gravel harvesting on river system, Case study: Shirud River Tonekabon (Mazandaran Province)

Today rivers have many benefits to human societies, but due to this exploitation, negative effects on the rivers have been created and the river system is out of balance. One of these benefits is the harvesting of sand from the floor and riverside boundaries, which has grown with increasing urbanization, which has adverse effects on the morphology and river ecosystem. One of the rivers whose sand and gravel harvesting have caused a morphological and environmental imbalance in Mazandaran province is the Shirud River in the city of Tonekabon, which originates from the Alborz mountains and flows into the Caspian Sea. Now in the plain section of the Shirud River, three sandy mines are active that harvest sand from the bed and the sides of the river. The first mine in Kashkou village, the second mine near the village of Solayman Abad and the third mine is located in the village of Kechanak. In this study, the morphology and changes in sand and gravel on the bed and side of the river were studied first through field observations. In the following, the effects of sand and gravel on river water quality were investigated. To do this, eight water sampling sites along the river were selected and in two dehydrated and high water samples were taken and water quality parameters including temperature, pH, TSS, TDS, electrical conductivity (EC), turbidity and hardness were measured.The statistical methods including one-sample t-test, Wilcoxon and Friedman were used to examining the differences in the values of each of the variables. Investigating the differences in the values of variables using the t one-sample test shows that the values of each of these variables in different sites show a significant difference from upstream downstream. Also, variations in morning and evening samples using the Wilcoxon test show that except for a sample of NTU and hardness of water, the amount of other variables has a significant difference. Examining each of the samples in the four times taken by using Friedman test indicates a distinct difference between all the variables in different samples. Despite the significant changes in each of the samples, the value of any of the variables is not more than the standard value and the river is in normal condition. The most important geomorphological outcomes of these observations are the downfall and fall of the river bed, coastal erosion and instability of the Shirud canal during winter floods.