What are primary causes of flooding in river systems?

The primary causes of flooding in river systems are indeed heavy rainfall, snowmelt, and human activities. Heavy rainfall is a significant factor because it can lead to the rapid accumulation of water in rivers, causing them to overflow. Snowmelt contributes to flooding in spring when temperatures rise, leading to the melting of snow in mountainous regions, which increases river flow.

Human activities play a critical role in flooding as well; for instance, urbanization can increase runoff due to the creation of impermeable surfaces, such as roads and buildings, which prevents water from being absorbed into the ground. Additionally, activities like deforestation and land-use changes can disrupt natural water drainage patterns, exacerbating the risk of flooding.

While other options might present valid causes of flooding, they mix elements that do not primarily contribute to flooding or include less relevant factors. For instance, agricultural runoff may exacerbate pollution problems but is not considered a primary cause of flooding itself.