Ans. A river cannot flow straight because when it flows, it either erodes something or deposits. When erosion occurs in a river course, it happens in an uneven manner as the bedrocks and the bank wall is composed of non-homogenous materials. On the other hand, when deposition takes place, it doesn’t occur throughout the course. In the areas where deposition took place, that region becomes shallow, and the river's flow gets obstructed due to the presence of sediment. Due to the reasons mentioned above, a river cannot flow in a straight course. Most of the rivers either flow in a sinuous course or meandering course. Whether a river is sinuous or meandering is determined by the sinuosity index, which is calculated by dividing the river's actual length by the shortest path. The value of the sinuosity index is between 1-1.5, known as a sinuous course, and more than 1.5 is known as a meandering course.