Rapids are sections of a river characterized by a significant and abrupt decrease in gradient, leading to highly turbulent flow and increased water velocity. Their formation is generally attributed to variations in the river's underlying geology, such as encountering bands of resistant bedrock or substantial changes in the riverbed material. The differential erosion of these varied materials creates constrictions and changes in the channel's cross-section, causing the water to speed up and churn. These turbulent environments are crucial physical features in river systems, influencing the transport of sediment and shaping the local aquatic ecology.