# Why reservoirs and drinking watersheds are surrounded by forest, and what the trees do for the water

> Why cities buy and keep woods around their drinking water, how forest soil, roots and shade protect it, how wide a stream buffer should be, and what to plant.

Published 2026-10-24. Tags: Forests & habitat, How trees work, What to plant. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/why-reservoirs-are-surrounded-by-forest/

So many reservoirs sit in forest because forested land gives the cleanest and steadiest water a town can get, and the cheapest to treat. A forest floor soaks up rain that pavement and bare ground would shed. Roots and leaf litter hold the soil, and trees take up nitrogen and phosphorus before they reach the water. Water suppliers therefore buy, keep and manage the woods around their reservoirs and the streams that feed them. Most of New York City's water, and the supply Massachusetts draws from its Quabbin and Wachusett reservoirs, reaches the tap unfiltered.

![Still lake mirroring white clouds, ringed by hardwoods in fall color down to the waterline, with lily pads in front](https://bluewhaletrees.com/img/articles/why-reservoirs-are-surrounded-by-forest-1-1000.jpg?v=29db6731 "A lake ringed by forest to the waterline on October 24, 2026. No land cover keeps water cleaner, or cheaper to treat, than forest.")
*Photo: Blue Whale Trees*

## Why the forest is there: the laws of 1897 and 1911

The national forests have served water since 1897. The Organic Administration Act of that year says the forest reserves, today's national forests, were established to secure favorable water flows.

In the East, national forests had to be bought from private owners. A flood made the case. In March 1907, floodwater raced down the Monongahela River and did some $100 million in damage. Congressman John Weeks of Massachusetts then proposed buying land near the headwaters of navigable streams and keeping it as forest for good, which let the purchases pass constitutional review. President Taft signed the Weeks Act on March 1, 1911.

Forested land yields more than half of the water that flows off the lower 48 states, and about 74 percent of drinking water is drawn from surface water such as streams and reservoirs. Forests are a source of clean drinking water for over 150 million people in the United States.

## Why water suppliers own the land around their reservoirs

New York City draws on 19 reservoirs and three controlled lakes in a watershed of nearly 2,000 square miles. They send one billion gallons a day to more than 8.8 million city residents and more than a million people nearby. The Catskill and Delaware supply, 91.3 percent of the water in 2025, is clean enough to go unfiltered. Only four other large cities in the country do not filter their surface water. To keep that status, the city spends over $100 million a year to conserve watershed land, manage forests and streams, and help farms keep pollution out.

Massachusetts runs its Quabbin, Wachusett, Ware River and Sudbury watersheds as one unfiltered supply for 2.7 million people in 53 communities. Over 100,000 acres of the watershed forest are protected from development. The state opens less than 1 percent of that forest a year so that young trees keep coming. More than 1,000 harvests in 50 years have shown no drop in water quality. To the agency that manages it, the forest does the work of a treatment plant:

> "Forested watersheds provide a living green bio-filter that functions 24 hours a day on free solar energy and is as reliable and efficient as the most responsibly engineered water treatment plant." (Massachusetts Department of Conservation and Recreation, *DCR Watershed Forestry Program*)

Forest also lowers the bill for treating water. A 2002 study of 27 water suppliers, by the Trust for Public Land and the American Water Works Association, measured how much. Every 10 percent more forest cover in the source area cut treatment and chemical costs by about 20 percent, up to about 60 percent forest cover.

## What the trees do for the water

A forest does five jobs for the water that drains from it:

- **Rain soaks in.** Under natural ground cover, about half of the rain soaks into the ground and a tenth runs off. In a highly developed watershed, about 15 percent soaks in and 55 percent runs off, carrying sediment and pollutants.
- **Soil stays put.** Runoff slows among the trees, and its sediment settles out, trapped by leaf litter, roots and porous forest soil. Much of the phosphorus in runoff is attached to soil particles, so it stays out of the water too. Deep roots hold the bank itself.
- **Nitrogen is taken up.** Roots store nutrient pollution in leaves and limbs, and in wet soil microbes turn nitrate into nitrogen gas. A band of forest along a stream can cut the nitrogen in shallow groundwater by as much as 80 percent. Excess nitrogen feeds toxic algal blooms, oxygen depletion and fish kills.
- **Shade keeps the water cool.** In summer, unshaded streams in Maryland can run 10 to 20 degrees warmer than forested ones. Cool water holds more oxygen, and warm water fuels harmful algae blooms.
- **Flows even out.** A forest catches the rain, stores it and lets it go slowly. Where the trees are lost, more water runs over the ground and storm flows rise.

![A band of hardwoods in fall color, one crown bright red, rising straight from the edge of a still lake that mirrors them](https://bluewhaletrees.com/img/articles/why-reservoirs-are-surrounded-by-forest-2-1000.jpg?v=1dd1386b "Forest meets the water on October 24, 2026. Leaf litter, roots and porous forest soil trap sediment before runoff reaches the water.")
*Photo: Blue Whale Trees*

## How wide a strip of trees the water needs

The strip of trees and shrubs along a stream or shore is a [stream buffer](https://bluewhaletrees.com/articles/glossary/index.html.md#stream-buffer). The federal conservation standard for one, in its October 2020 edition, sets these widths, measured from the water's edge on one side:

| Width from the water | What it does |
| --- | --- |
| 35 feet | The minimum to trap sediment washing off the land |
| 50 feet | The minimum to filter nutrients and pesticides, and room for amphibians and edge birds |
| 100 feet | Room for birds of the forest interior and small mammals |
| 165 feet | Room for large mammals |

Wider strips catch more nitrogen. A 2005 review of buffer studies by EPA researchers found that buffers removed about three quarters of the nitrogen passing through them on average, and forest did better than grass. Buffers wider than about 165 feet removed it consistently, while a few strips under 50 feet wide added nitrogen instead.

A buffer works best in three zones. Deep-rooted trees and shrubs at the water's edge hold the bank and shade the stream. A band of managed forest upslope takes up nutrients and traps sediment. A grass strip on the outside slows and spreads runoff, and it can remove half or more of the sediment and phosphorus before the water reaches the trees.

![Aerial view of a creek winding through farm fields, lined with tall trees, with strips of tan grass along the field edges](https://bluewhaletrees.com/img/articles/why-reservoirs-are-surrounded-by-forest-3-1000.jpg?v=5a409a61 "Trees line Bear Creek in Story County, Iowa, with strips of grass between them and the fields. The grass slows and spreads runoff before it reaches the trees.")
*Photo: USDA, [Public domain](https://commons.wikimedia.org/wiki/File%3ARiparian_buffer_on_Bear_Creek_in_Story_County%2C_Iowa.JPG) ([source](https://commons.wikimedia.org/wiki/File%3ARiparian_buffer_on_Bear_Creek_in_Story_County%2C_Iowa.JPG))*

## Native trees and shrubs that hold a bank

Every plant below grows wild on stream banks and takes wet ground. Blue Whale Trees grows all of them from seed. For the water's edge and a sunny bank:

- [Bald cypress](https://bluewhaletrees.com/plants/bald-cypress/index.html.md), which grows in standing water or on a dry lawn, for a pond edge or a low spot that floods each spring. Zones 4 to 10, 50 to 80 feet.
- [Swamp white oak](https://bluewhaletrees.com/plants/swamp-white-oak/index.html.md), an oak of stream banks and lake edges that takes spring flooding better than most oaks. Zones 3 to 8, 50 to 60 feet.
- [River birch](https://bluewhaletrees.com/plants/river-birch/index.html.md), planted in streamside buffers to reduce bank erosion, which takes floods of a few weeks. Zones 3 to 9, 40 to 70 feet.
- [Peachleaf willow](https://bluewhaletrees.com/plants/peachleaf-willow/index.html.md), whose fast, deep roots protect riverbanks from erosion and should stay clear of drains and septic fields. Zones 2 to 8, 35 to 60 feet.
- [Smooth alder](https://bluewhaletrees.com/plants/smooth-alder/index.html.md), whose roots fix nitrogen and knit wet banks together. Zones 4 to 9, 10 to 20 feet.
- [Buttonbush](https://bluewhaletrees.com/plants/buttonbush/index.html.md), which grows in water up to 3 feet deep and forms dense stands that hold shorelines. Zones 4 to 10, 5 to 12 feet.
- [Silky dogwood](https://bluewhaletrees.com/plants/silky-dogwood/index.html.md), which keeps a stream bank from washing out after two or three years. Zones 4 to 8, 6 to 12 feet.
- [Red osier dogwood](https://bluewhaletrees.com/plants/red-osier-dogwood/index.html.md), whose roots hold the soil of a pond edge or stream bank. Zones 2 to 7, 6 to 12 feet.
- [Black elderberry](https://bluewhaletrees.com/plants/black-elderberry/index.html.md), which holds moist soil against erosion beside streams and ponds. Zones 3 to 9, 6 to 12 feet.

Under existing trees, in part shade or shade:

- [American hornbeam](https://bluewhaletrees.com/plants/american-hornbeam/index.html.md), the understory tree of stream bottoms, which takes flooding and deep shade. Zones 3 to 9, 20 to 35 feet.
- [Spicebush](https://bluewhaletrees.com/plants/spicebush/index.html.md), which quickly holds the soil on a stream bank and grows in deep shade. Zones 4 to 9, 6 to 12 feet.
- [Sweet pepperbush](https://bluewhaletrees.com/plants/sweet-pepperbush/index.html.md), whose suckering roots form a low thicket that helps hold the bank. Zones 4 to 9, 4 to 8 feet.

## What to do on your own shore or stream bank

Streamside forest has disappeared along many rivers and streams. Without it, runoff reaches the water faster and carries more sediment and nutrients. Five steps put some of it back:

1. **Stop mowing short of the water.** Let a strip at least 35 feet wide grow up in native trees and shrubs, wider where you can.
2. **Plant in zones.** Put the deep-rooted plants at the water's edge and more trees upslope. Keep a grass strip where a lawn or field drains toward the water.
3. **Match each plant to the water.** The [tree finder](https://bluewhaletrees.com/tools/tree-finder/index.html.md?moisture=wet&want=erosion-control) lists the trees and shrubs we grow for wet ground and erosion control. The [planting-site check](https://bluewhaletrees.com/tools/site-check/index.html.md?species=river-birch&moisture=wet) judges one species on one spot.
4. **Leave the leaves.** [Leaf litter](https://bluewhaletrees.com/articles/leaf-litter-not-wood-chips/index.html.md) traps soil, and the leaves that fall into the water feed the stream's food chain.
5. **Fix what drains in.** Pollution washed off the land, including runoff from lawns and leachate from septic systems, is the leading cause of poor water quality, and fertilizer adds nitrogen.

A strip of trees cannot make up for everything above it:

> "A buffer alone cannot fully compensate for unmanaged runoff upstream." (Maryland Forest Service, *Riparian Forest Buffers*)

For a corner that floods every spring, the [Wet Spot Pack](https://bluewhaletrees.com/bundles/wet-spot/index.html.md) sets six natives together, among them bald cypress, smooth alder and silky dogwood. The shop's list of [trees and shrubs for wet soil](https://bluewhaletrees.com/shop/wet-soil/index.html.md) holds every one we grow that takes wet ground. [Which natives take standing water](https://bluewhaletrees.com/articles/trees-that-soak-up-water/index.html.md) has a guide of its own, and [protecting forestland for good](https://bluewhaletrees.com/articles/protecting-forestland-for-good/index.html.md) covers keeping your own woods as forest.

## Common questions

### Why are reservoirs surrounded by trees?

Forest keeps the water cleanest and cheapest to treat: its soil soaks up rain, its roots hold the soil and its trees take up nutrients. Water suppliers buy and protect the woods around their reservoirs for that reason.

### Do trees clean drinking water?

They keep sediment and nitrogen out of it, so water from forested land needs less treatment. In a 2002 study of 27 water suppliers, each 10 percent more forest cover cut treatment and chemical costs by about 20 percent, up to about 60 percent cover.

### How wide should a buffer of trees be along a stream?

At least 35 feet from the water to trap sediment, and 50 feet to filter nutrients and pesticides. Wider strips remove nitrogen more consistently.

### What trees should I plant next to a pond or lake?

Near the water, plant trees that take wet ground, such as bald cypress, river birch, swamp white oak and peachleaf willow. At the edge itself, buttonbush, silky dogwood and smooth alder hold the bank.

## Sources

- [Forests to Faucets 2.0: Connecting Forests, Water, and Communities](https://research.fs.usda.gov/download/treesearch/63723.pdf): USDA Forest Service, General Technical Report WO-99 (Erika Mack, Rebecca Lilja, Sally Claggett, Ge Sun and Peter Caldwell, 2022): forests as a source of clean drinking water for over 150 million people, forested land yielding more than half of the water of the lower 48 states, 74 percent of drinking water withdrawals from surface water, and the overland flow and stormflow that follow the loss of forest cover.
- [Watershed Focus Areas](https://www.fs.usda.gov/naturalresources/watershed/focus_areas.shtml): USDA Forest Service, Watershed Program: the 1897 Organic Administration Act's purpose for the forest reserves, favorable conditions of water flows.
- [Passing the Weeks Act](https://foresthistory.org/research-explore/us-forest-service-history/policy-and-law/the-weeks-act/passing-weeks-act/): Forest History Society: a federal law needed to buy private land for watersheds, the Monongahela flood of March 1907, Weeks's bill of 1908 to buy land near the headwaters of navigable streams, its amendment keeping that land as forest for good and its standing under the commerce clause, and Taft's signature on March 1, 1911.
- [New York City Drinking Water Supply and Quality Report 2025](https://www.nyc.gov/assets/dep/downloads/pdf/water/drinking-water/drinking-water-supply-quality-report/2025-drinking-water-supply-quality-report.pdf): New York City Department of Environmental Protection: the 19 reservoirs and three controlled lakes, the watershed of nearly 2,000 square miles, one billion gallons a day, the unfiltered Catskill and Delaware supply, the five large unfiltered cities, and over $100 million a year on watershed programs.
- [DCR Office of Watershed Management](https://www.mass.gov/orgs/dcr-office-of-watershed-management): Massachusetts Department of Conservation and Recreation: the four watersheds that supply 2.7 million people in 53 communities without filtration.
- [DCR Watershed Forestry Program](https://www.mass.gov/info-details/dcr-watershed-forestry-program): Massachusetts Department of Conservation and Recreation: over 100,000 acres of protected watershed forest, openings on less than 1 percent of it a year, more than 1,000 harvests with no drop in water quality, the forest that catches, stores and slowly releases rain, forest cover and water quality, and the quotation.
- [Protecting the Source: Land Conservation and the Future of America's Drinking Water](https://www.tpl.org/wp-content/uploads/2014/03/water-protecting_the_source_final.pdf): Trust for Public Land and American Water Works Association (Caryn Ernst, 2004): the 2002 study of 27 water suppliers and its forest cover and treatment costs, infiltration and runoff under natural cover and in a developed watershed, and lawns and septic systems among the sources of water pollution.
- [Stream Buffers](https://www.chesapeakebay.net/issues/whats-at-risk/stream-buffers): Chesapeake Bay Program: runoff slowed by trees, roots storing nutrients in leaves and limbs, deep roots holding stream banks, cool water holding more oxygen, warm water fueling algae blooms, leaf litter at the base of the stream's food chain, and buffers lost along many rivers and streams.
- [Riparian Forest Buffers](https://dnr.maryland.gov/forests/Pages/publications/buffers.aspx): Maryland Department of Natural Resources, Maryland Forest Service: sediment settling out in a forested buffer, phosphorus attached to soil particles, nitrate turned to nitrogen gas, the three zones and what each removes, unshaded streams 10 to 20 degrees warmer, faster and dirtier runoff without a buffer, shorelines among the places buffers protect, and the quotation.
- [Riparian Forest Buffer, Conservation Practice Standard Code 391](https://www.nrcs.usda.gov/sites/default/files/2022-09/Riparian_Forest_Buffer_391_CPS_10_2020.pdf): USDA Natural Resources Conservation Service (October 2020): the 35-foot and 50-foot minimum widths measured from the water on one side, the 50-, 100- and 165-foot widths for wildlife, and native trees and shrubs.
- [Riparian Buffer Width, Vegetative Cover, and Nitrogen Removal Effectiveness: A Review of Current Science and Regulations](https://www.epa.gov/sites/default/files/2019-02/documents/riparian-buffer-width-2005.pdf): U.S. Environmental Protection Agency, EPA/600/R-05/118 (Paul Mayer, Steven Reynolds, Marshall McCutchen and Timothy Canfield, 2005): buffers removing about three quarters of the nitrogen on average, forest buffers outperforming grass, consistent removal in buffers wider than 50 meters, narrow strips that sometimes added nitrogen, microbial denitrification, and fertilizer and the algal blooms, oxygen depletion and fish kills that excess nitrogen brings.

## Related

- Next step: [Open the tree finder](https://bluewhaletrees.com/tools/tree-finder/index.html.md?moisture=wet&want=erosion-control)
- [Trees and shrubs that soak up water: what they can do for a wet yard, and the natives for a rain garden](https://bluewhaletrees.com/articles/trees-that-soak-up-water/index.html.md)
- [How floodplain trees survive high water](https://bluewhaletrees.com/articles/how-floodplain-trees-survive-high-water/index.html.md)
- [Tree roots run shallow and wide, not deep: what that means for watering, mulch and the house](https://bluewhaletrees.com/articles/tree-roots-run-shallow-and-wide/index.html.md)
- [Protecting forestland for good: conservation easements and the other tools](https://bluewhaletrees.com/articles/protecting-forestland-for-good/index.html.md)