--- title: "Drainage Basins and Why Rivers Flood" description: "A drainage basin collects water into a river system. Rain can be stored, evaporate or reach channels by surface and underground routes. River response depends on these routes, rainfall and basin condi" 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Drainage Basins and Why Rivers Flood A drainage basin collects water into a river system. Rain can be stored, evaporate or reach channels by surface and underground routes. River response depends on these routes, rainfall and basin conditions. Paving and drains can speed runoff downstream. 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 - Use stated peak rainfall and discharge times to compare lag times, and explain why a short lag shows a rapid river response - 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 the land drained by a river and its tributaries. A drainage divide, usually a ridge of high ground, separates neighbouring surface drainage basins. Not all rain reaches a river: some evaporates, is used by plants or remains stored. Groundwater boundaries need not match the surface divide. Water can reach a channel as surface runoff, throughflow through soil, or groundwater flow through rock and sediment. Surface runoff often gives a rapid storm response; underground routes commonly deliver water more slowly. Infiltration means water entering the ground. Saturated, frozen or sealed ground can limit infiltration. Even gentle rain can then produce runoff. *(drawing: This shows one route. Other water may run over the surface or return to the air.)* **Example.** A flood hydrograph plots discharge, the volume of river water passing each second, against time. Here, lag time means the interval from peak rainfall to peak discharge. Peak rain at 3 pm and peak discharge at 7 pm give a four-hour lag. A rapid rise and short lag show a quick response, not its exact cause. Flooding also depends on whether flow exceeds channel capacity. Paving reduces infiltration. Drains without storage can deliver runoff quickly to channels, increasing flood risk downstream. Removing trees can reduce interception and increase runoff, depending on soil and rainfall. Channel straightening can speed water onward. Floodplain building exposes people to flooding. Storage and drainage measures can change a development's effect on runoff. **Recap.** Rainfall, storage and water routes shape a river's response. A short lag means a quick response; flood risk also depends on channel capacity and development. *Lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Optional connections - [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)