---
title: "Drainage Basins and Why Rivers Flood"
description: "Everything that falls on a basin has to leave it, and how fast it gets to the channel decides whether the river rises gently or all at once. Concrete, drains and cleared hillsides all shorten that jou"
canonical: https://lightmysky.com/learn/geography/drainage-basins-and-why-rivers-flood-mt_Vu4fTs7goJ
source: https://lightmysky.com/learn/geography/drainage-basins-and-why-rivers-flood-mt_Vu4fTs7goJ.md
retrieved: 2026-09-12
---

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# Drainage Basins and Why Rivers Flood

Everything that falls on a basin has to leave it, and how fast it gets to the channel decides whether the river rises gently or all at once. Concrete, drains and cleared hillsides all shorten that journey.

Subject: Geography · Area: Physical Geography · Ages 14 to 16
Page: https://lightmysky.com/learn/geography/drainage-basins-and-why-rivers-flood-mt_Vu4fTs7goJ

## Ready when they can

- Explain what a drainage basin is and name three routes water takes to reach the channel
- Read a flood hydrograph and say what a short lag time tells you about the basin
- Give two human changes that raise flood risk downstream and say why each does

## Lesson: Why some rivers flood in a flash

A drainage basin is all the land that drains into one river, fenced by high ground called the divide. Rain inside the fence ends up in the same channel. So whatever happens anywhere in the basin shows up downstream at the river gauge.

Water reaches the channel by three routes. Some runs straight over the surface, some creeps through the soil, and some sinks deep to groundwater that feeds the river slowly. Hard or frozen ground sends most water overland, and then the river spikes.

**Example.** A flood hydrograph plots flow against time after rain. A short gap between peak rain and peak flow means water rushed in fast, hinting at steep slopes, thin soils or paved streets. A tall narrow peak warns of a flashy river that floods with little notice.

**Tip.** People shorten the journey. Concrete and tarmac seal the soil and shunt storm water to drains. Straightened channels rush water onward, cleared slopes shed rain that trees would have caught, and homes on floodplains sit exactly where full rivers go.

**Recap.** Basin shape and soil set the speed of water, the hydrograph reveals it, and paving and clearing make floods faster and taller.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Reading a Landscape Off a Map](https://lightmysky.com/learn/geography/reading-a-landscape-off-a-map-mt_MWlY_e59Rg)
- [From Source to Sea](https://lightmysky.com/learn/geography/from-source-to-sea-mt_VuUB9PiYan)

## Opens up

- [Choosing How to Defend a Coast](https://lightmysky.com/learn/geography/choosing-how-to-defend-a-coast-mt_3Px4N4va3o)