1. Rivers and Drainage Basins - A Prelude to Flooding Drainage basin/River basin: An area of land drained by a main river and its tributaries. Drainage basin/River basin: An area of land drained by a main river and its tributaries. Watershed: The river’s boundary, marked by ridge of higher land. It separates one drainage basin from neighboring drainage basins. Watershed: The river’s boundary, marked by ridge of higher land. It separates one drainage basin from neighboring drainage basins.

2. Watershed

4. Rivers and Drainage Basins - A Prelude to Flooding Drainage basins can store rainwater within the river itself, in lakes, and in the ground. Drainage basins can store rainwater within the river itself, in lakes, and in the ground. 97% of the world’s water is stored in the seas/oceans. 97% of the world’s water is stored in the seas/oceans. 2% of the world’s water is stored as ice and snow in arctic/alpine areas. 2% of the world’s water is stored as ice and snow in arctic/alpine areas. 1% is fresh water on land or water vapor in the air.1% is fresh water on land or water vapor in the air.

6. Rivers and Drainage Basins - A Prelude to Flooding Hydrological Cycle: The constant recycling of water between the sea, air, and land. No water is lost from the cycle; thus, it is a closed system. Hydrological Cycle: The constant recycling of water between the sea, air, and land. No water is lost from the cycle; thus, it is a closed system. Inputs Flows Stores Flows Outputs System

7. Hydrological Cycle

8. Rivers and Drainage Basins - A Prelude to Flooding A drainage basin is part of the hydrological cycle, but is an open system. A drainage basin is part of the hydrological cycle, but is an open system.

9. Rivers and Drainage Basins - A Prelude to Flooding River flooding typically occurs during heavy storms when SURFACE RUN-OFF is great, or when the soil becomes over SATURATED and water rises above the WATER TABLE. River flooding typically occurs during heavy storms when SURFACE RUN-OFF is great, or when the soil becomes over SATURATED and water rises above the WATER TABLE. In all cases of flooding and non- flooding, the water works its way to the sea/ocean where it is lost to the system.