Rivers and Drainage BasinsRivers and Drainage Basins- A Prelude to Flooding- A Prelude to Flooding

• Drainage basin/River basinDrainage basin/River basin: An area of : An area of land drained by a main river and its land drained by a main river and its tributaries. tributaries.

• WatershedWatershed: The river’s boundary, : The river’s boundary, marked by ridge of higher land. It marked by ridge of higher land. It separates one drainage basin from separates one drainage basin from neighboring drainage basins.neighboring drainage basins.

WatershedWatershed

Rivers and Drainage BasinsRivers and Drainage Basins- A Prelude to Flooding- A Prelude to Flooding

• Drainage basins can store rainwater within Drainage basins can store rainwater within the river itself, in lakes, and in the ground.the river itself, in lakes, and in the ground.

• 97% of the world’s water is stored in the 97% of the world’s water is stored in the seas/oceans.seas/oceans.

• 2% of the world’s water is stored as ice and 2% of the world’s water is stored as ice and snow in arctic/alpine areas.snow in arctic/alpine areas.

•1% is fresh water on land or water vapor in 1% is fresh water on land or water vapor in the air.the air.

Rivers and Drainage BasinsRivers and Drainage Basins- A Prelude to Flooding- A Prelude to Flooding

• Hydrological CycleHydrological Cycle: The constant : The constant recycling of water between the sea, air, recycling of water between the sea, air, and land. No water is lost from the and land. No water is lost from the cycle; thus, it is a cycle; thus, it is a closed systemclosed system..

Inputs Flows FlowsStores Stores Flows Outputs

SystemSystem

Hydrological CycleHydrological Cycle

Rivers and Drainage BasinsRivers and Drainage Basins- A Prelude to Flooding- A Prelude to Flooding

• A drainage basin is part of the A drainage basin is part of the hydrological cycle, but is an hydrological cycle, but is an open system.open system.

Rivers and Drainage BasinsRivers and Drainage Basins- A Prelude to Flooding- A Prelude to Flooding

• River flooding typically occurs during River flooding typically occurs during heavy storms when SURFACE RUN-OFF heavy storms when SURFACE RUN-OFF is great, or when the soil becomes over is great, or when the soil becomes over SATURATED and water rises above the SATURATED and water rises above the WATER TABLE.WATER TABLE.

• In all cases of flooding and non-In all cases of flooding and non-flooding, the water works its flooding, the water works its way to the way to the sea/ocean where it is lost to the system.sea/ocean where it is lost to the system.