The direction of water supply in the wealthy, water-stressed world has quietly shifted: the next unit of water increasingly comes from reusing water that has already been used once, which lets a country build fewer new reservoirs to cover the same demand. The reasons are practical before they are ecological. The best dam sites are already dammed; a reservoir is a depreciating asset that silts up and evaporates; and the ecological bill of impounding a river — fragmented habitat, starved deltas, methane, drowned valleys — is now measured, not guessed at. Water reuse sidesteps all of it. It adds a rainfall-independent supply that grows with the population instead of the weather, and it does so without flooding another valley or cutting another river in two. This piece sets out why the trend is toward reusing water and building fewer new reservoirs — and why that is the ecosystem-preserving path.
Key takeaways
- The large-dam-building era has peaked, and removals are at a record pace. In 2024 US communities removed 108 dams — tied for the most in any year on record — reconnecting about 2,528 miles of river; more than 2,350 dams have come down since 1912. The country finished tearing out four dams on the Klamath River in 2024, the largest dam removal in US history, reopening salmon habitat closed for over a century.
- A reservoir is a depreciating asset. A 2023 UN University-led assessment of large dams across 150 countries found they have already lost an estimated 13–19% of their original storage to trapped sediment, headed for roughly 26% by 2050. On top of that, evaporation is relentless: across the Colorado system about 1.9 million acre-feet — some 13% of the river — evaporates every year, with Lake Mead alone losing 600,000–860,000 acre-feet.
- Reuse is a proven, rainfall-independent supply. Israel reuses close to 90% of its municipal wastewater — the world's highest rate — mostly for farming; Singapore's NEWater meets up to 40% of national demand; Orange County's system purifies 130 million gallons a day; and Windhoek, Namibia, has drunk recycled water since 1968.
- Reuse is the ecosystem-preserving option. It keeps the demand loop inside the city, so rivers stay in place, sediment keeps reaching deltas, fish keep migrating, and no new valley is drowned — freeing water to be left in the environment rather than taken from it.
- The honest limit: reservoirs are not obsolete. Reuse recycles water that rivers and reservoirs still store and deliver, and much of the developing world is rightly still building storage. The shift is where new supply comes from in mature, water-short economies — a build-or-reuse call that only open data can settle, which is what the Reservoir Transparency Index scores.
1. The dam-building era has peaked — and removals are at a record pace
For most of the twentieth century, "more water" meant "more concrete." Then the rich world ran out of good places to put it. In the United States and Western Europe, the era of large-dam construction effectively ended decades ago: the productive sites were taken, the cheap water was captured, and the political tolerance for drowning valleys evaporated.
What replaced it is the opposite motion. The United States now removes dams at a record pace — 108 in 2024, tied for the most ever, reconnecting roughly 2,528 miles of river, according to American Rivers, which counts more than 2,350 removals since 1912. Most are small, obsolete or unsafe structures, but the flagship is not small: in 2024 crews finished demolishing four hydroelectric dams on the Klamath River, the largest dam-removal project in US history, letting salmon swim a river they had been locked out of for over a century.
The driver is economic, not ideological. A dam is worth building only when the water and power it captures exceed what it costs and destroys; in basins that are already heavily dammed, that equation has stopped clearing, and the marginal reservoir now looks worse than the alternative — using the water already in the system more than once.
2. A reservoir is a depreciating asset
The case against "just build another one" is strongest where it is least intuitive: a reservoir does not hold its value. It silts up, and it evaporates.
Sedimentation is the slow leak. Every river carries sediment; a dam stops it, and the trapped silt steadily fills the space meant for water. A 2023 assessment led by the UN University's water institute, covering large dams in 150 countries, estimated they have already lost 13–19% of their original combined storage and will be down roughly 26% by 2050 — global storage falling from about 6,316 to 4,665 billion cubic metres. A new dam starts losing capacity the day it closes its gates, and unlike a recycling plant, you cannot expand it back.
Evaporation is the fast leak, and it is brutal in exactly the arid places that build reservoirs to fight drought. Across the Colorado River system, roughly 1.9 million acre-feet — about 13% of the entire river's water — is lost to evaporation each year; Lake Mead alone gives up an estimated 600,000–860,000 acre-feet, and Lake Powell hundreds of thousands more. A big open reservoir in the desert is, in part, a machine for handing water back to the sky.
The reservoirs this site tracks make the depreciation visible. Mead and Powell have sat near record lows for years — not because the West stopped storing water, but because a hotter, drier basin plus evaporation plus a fixed structure is a losing combination. Purified wastewater loses nothing to a lakebed or a hot afternoon.
3. The ecosystem bill a dam runs up
Preserving the ecosystem is not a soft add-on to this argument; it is a large, now-quantified cost that a new reservoir imposes and reuse does not.
- A dam cuts a river in two. It blocks the migration of fish and the movement of everything that depends on a connected river, which is precisely why dam removal — Klamath, and the earlier Elwha removals in Washington — is now treated as ecological restoration worth hundreds of millions of dollars.
- A dam starves the coast. By trapping sediment, a dam stops the silt that builds and defends river deltas. Downstream, deltas sink and shorelines retreat because the sediment that used to renew them is sitting behind a wall.
- A dam floods a living valley. New storage means inundating land — habitat, farmland, and often communities — permanently.
- A dam emits greenhouse gases. Decomposing vegetation under reservoir water releases methane. The world's reservoirs emit roughly 1 gigaton of CO₂-equivalent a year — about 1.3% of human-caused greenhouse emissions, most of it methane (Deemer et al., BioScience, 2016). A "clean" reservoir is not automatically a low-carbon one.
Reuse carries none of these costs. It fragments no river, buries no valley, and traps no sediment. Its footprint is a treatment plant on land already zoned for infrastructure.
4. Why reuse scales with population, not rainfall
The structural reason reuse is gaining share is that it scales with the thing you can count on — people — instead of the thing you cannot — rainfall. Every city produces wastewater every day, in proportion to its population. That makes reused water a drought-resilient, rainfall-independent supply: a source that barely shrinks in the dry year you need it most. A reservoir does the reverse — it is emptiest exactly when the drought is worst.
And it is not experimental. It is decades of operating record:
- Israel treats and reuses close to 90% of its municipal wastewater — roughly four times the next country (Spain, about 20%) — overwhelmingly to irrigate farmland, freeing freshwater for cities.
- Singapore's NEWater — recycled water pushed through microfiltration, reverse osmosis and UV — meets up to about 40% of national water demand today, with a target of 55% by 2060.
- Orange County, California runs the world's largest advanced water-purification plant for potable reuse. Since its 2023 final expansion it produces 130 million gallons a day — enough for nearly a million people — recharging the groundwater the county drinks from.
- Windhoek, Namibia, has practised direct potable reuse — sewage back to tap water — since 1968, the first city in the world to do it, still supplying about a third of its drinking water that way.
The regulatory frame is catching up to the engineering. In December 2023, California adopted nation-leading rules for direct potable reuse — purified wastewater sent straight into the drinking-water system — and they took effect on 1 October 2024. The question has shifted from whether recycled water is safe to how fast utilities can build the plants.
5. Why reuse is the ecosystem-preserving choice
Put the two paths side by side. To add supply with a reservoir, you take more water out of a river and impound it, with all the fragmentation, sediment-trapping, flooding and methane that entails. To add supply with reuse, you take water that a city has already withdrawn, clean it, and use it again — closing a loop instead of tapping a new source.
That closed loop is what spares the ecosystem. Water reused inside the city is water that does not have to be pulled from a river, an aquifer or a new dam — which means it can instead be left in the environment as the flow that keeps a river alive, or never removed from the ground at all. Recycled and desalinated water are also what let a city lean less on any single dam, so that a reservoir which has become redundant can be retired rather than replaced. That is a different claim from tearing down the storage a region still needs: the aim is to shrink the demand for the next dam, not to empty the ones that work.
6. The honest limits
Reuse is a trend, not a miracle, and overselling it would be its own kind of dishonesty.
- It costs energy. Reverse osmosis is power-hungry, and advanced purification concentrates a briny waste stream that has to be disposed of. Reuse trades a land-and-ecology cost for an energy cost — a better trade in most water-short cities, but a real one.
- Reservoirs are not going away. Reuse recycles water that rivers and reservoirs still capture, store and deliver; dams also provide hydropower and flood control that a treatment plant does not. The point is not to empty the reservoirs but to stop assuming the only way to add water is to build another.
- The developing world is a different case. Countries still building basic water security — much of South Asia and Africa — often genuinely need new storage, and are right to build it. The reuse-over-reservoirs shift is sharpest in mature, water-stressed economies that already have all the dams a basin can sensibly hold.
None of this is knowable without open data, which is the argument this site exists to make: whether a country needs another reservoir or should reuse what it has is a question you can only answer honestly with a long, public record of how full its existing storage actually runs. We make that case in full in why reservoir data must be public, and score countries on it in the Reservoir Transparency Index.
The bottom line
The trend is real and it is directional: in the water-short rich world, new supply is coming from reuse, large-dam construction has largely stopped, and removals are running at a record pace. It is driven first by hard economics — reservoirs silt up, evaporate and have no good sites left — and it lands, not coincidentally, on the outcome an ecosystem would choose: rivers left connected, deltas fed, valleys unflooded, and water counted twice rather than pulled from a new source. You can watch the pressure that is forcing the change on our data pages for the United States, where Lake Mead and Lake Powell show what a depreciating reservoir looks like in real time.
FAQ
Is water reuse actually replacing reservoirs? Not one-for-one, and not everywhere. Reservoirs still store and deliver most of the world's managed water. What is changing is where the next unit of supply comes from: in mature, water-stressed economies it increasingly comes from reusing wastewater rather than building a new dam, because the good dam sites are gone and reused water is cheaper on the ecological ledger and independent of rainfall.
Why not just build more reservoirs to fight drought? Because a reservoir is emptiest exactly when the drought is worst, loses capacity every year to sediment (large dams are on track to lose about 26% of storage by 2050), and hands large volumes back to evaporation — roughly 13% of the entire Colorado River each year. Reused water suffers none of these, and it grows with the population instead of shrinking with the weather.
Is recycled water safe to drink? Yes, where it is properly treated and regulated. Advanced purification — microfiltration, reverse osmosis and UV — produces water that meets or exceeds drinking-water standards. Windhoek, Namibia, has done direct potable reuse since 1968, and in 2024 California's nation-leading rules for it took effect. Most reused water is used for agriculture, industry and groundwater recharge rather than sent straight to the tap.
How does reuse help the ecosystem specifically? Every litre a city recycles is a litre it does not have to take from a river, an aquifer or a new dam — leaving that water in the environment as the flow rivers need, and avoiding the fragmented habitat and trapped sediment a new dam would create. It also lets a city depend less on any single dam, which can make retiring a redundant one easier. (Large removals like the Klamath — the biggest in US history, completed in 2024 — took down ageing hydroelectric dams for salmon, a related but distinct trend.)
Does this mean dams are bad? No. Dams provide storage, hydropower and flood control, and much of the developing world still needs to build them. The argument is narrower: in places that already have all the reservoirs a basin can sensibly hold, reusing water is usually the better next step than damming another river — and it happens to be the one that preserves the ecosystem.
Sources: dam removals and Klamath — American Rivers (2024). Sedimentation — Perera et al., "Present and Future Losses of Storage in Large Reservoirs Due to Sedimentation," Sustainability (UNU-INWEH, 2023). Colorado River evaporation — US Bureau of Reclamation-linked research. Reservoir greenhouse gases — Deemer et al., BioScience (2016). Reuse figures — Israel Water Authority; Singapore PUB (NEWater); Orange County Water District (GWRS); City of Windhoek; California State Water Resources Control Board (direct potable reuse regulations, adopted December 2023, effective 1 October 2024). Live reservoir levels are our own, from the sources documented in how we get our data.