Are Reservoirs Cleaner Than Natural Lakes?

No. Across 5,696 graded lakes checked against the national dam inventory, 57% of natural lakes earn an A or B on nutrients and algae, against 34% of dam-built reservoirs. The reservoirs that do grade well are the interesting part — 289 of them hold an A.

The two groups, graded the same way

Every lake here is scored on the same three measurements — water clarity, total phosphorus, and chlorophyll-a — against the norms for its ecoregion. Nothing in the grade knows whether a dam is involved. The split below comes from matching each lake against the National Inventory of Dams: a lake with a co-located dam is counted as a reservoir, and a lake that was searched for and came back with no dam is counted as natural. Lakes that have not been through that match yet appear in neither column — a lake nobody has checked is unknown, not natural.

Grade	Reservoirs	% of reservoirs	Natural lakes	% of natural
A	289	18.9%	851	20.4%
B	224	14.7%	1,528	36.7%
C	346	22.6%	668	16.0%
D	377	24.7%	773	18.6%
F	293	19.2%	347	8.3%
A or B	513	33.6%	2,379	57.1%

Unrated lakes are excluded from both columns rather than counted as failures — an unmeasured lake is unknown, not an F. How the letter is assigned is in Lake Water Quality Grades Explained.

Why the gap exists

A reservoir is a river with a wall across it, so it inherits the river's watershed. Every acre upstream drains through it: cropland, pasture, stormwater, treated effluent. A natural lake in a closed basin may collect runoff from a few square miles; an impoundment can collect it from thousands. That is the largest single difference between the two columns, and it is a difference in what the water receives rather than in what the dam does.

Shape compounds it. Impoundments tend to be broad and shallow for their surface area, which keeps nutrient-rich bottom sediment inside the sunlit layer where algae can use it, and lets wind re-suspend it. Deep, steep-sided natural basins stratify in summer and hold their phosphorus below the thermocline instead. That mechanism is the subject of Do Deeper Lakes Have Better Water Quality?, and the nutrient side of it is in Why Lakes Turn Green.

There is also a siting effect that no amount of care can remove from this table. Dams get built where rivers, farms, and towns already are. The reservoir column is therefore partly a land-use column, and the honest reading is that impounded water tends to sit in harder places — not that impounding water is what ruined it.

What reservoirs are built for

The dam records carry a primary purpose, and it is not mostly flood control. Of the 2,236 matched reservoirs with a stated primary purpose, recreation is the largest single category — these are water bodies built to be used.

Primary purpose	Reservoirs	Share
Recreation	1,083	48%
Flood Risk Reduction	348	16%
Water Supply	341	15%
Irrigation	216	10%
Hydroelectric	148	7%
Other	100	4%

Age varies enormously. The oldest dam matched in this set is Damon Pond Dam, completed in 1760, behind Damon Pond in Massachusetts. A century of accumulated sediment is a different starting point from a reservoir filled in the 1980s.

The reservoirs that grade A

289 matched reservoirs hold an A, and 347 natural lakes hold an F. Neither category is destiny. The A-graded impoundments below are typically fed by forested rather than farmed watersheds, which is the same variable doing the work in both directions.

Reservoir	State	Grade	Clarity (ft)	Built for
Clearwater Lake	Minnesota	A	10	Other
Pelican Lake	Minnesota	A	16.4	Recreation
Pelican Lake	Minnesota	A	14.3	Other
Round Lake	Minnesota	A	10	Recreation
Washburn Lake	Minnesota	A	14.1	Recreation
Long Lake	Minnesota	A	19.5	Recreation
Long Lake	Minnesota	A	11.2	Recreation
Deer Lake	Minnesota	A	13.1	Recreation

What to do with this

Do not rule out a reservoir. Rule out a bad grade. The category shifts the odds and the individual letter settles the question, so read the lake's own report card, its clarity trend, and any current advisory before a swim or a cabin booking. The state-by-state reservoir listings are under reservoirs by state, the top-graded water overall is in the cleanest ranking, and the buyer's version of this question is in What to Check Before Buying Lakefront Property.

Frequently Asked Questions

Not in this dataset. Of 1,529 graded reservoirs, 34% earn an A or B on nutrients and algae. Of 4,167 graded natural lakes checked against the same dam inventory, 57% do — a gap of about 24 percentage points. Reservoirs are also about 2.3 times as likely to grade an F.

A reservoir sits where a river used to run, so it collects whatever the whole upstream watershed sends down it — fertilizer, eroded soil, treated wastewater. Many are shallow relative to their surface area, which keeps nutrient-rich sediment within reach of sunlight and wind. Natural lakes more often sit in closed basins with far smaller drainage areas feeding them.

Often, yes — a grade is a summary of nutrient and algae measurements over time, not a same-day clearance, and plenty of reservoirs grade A. Check the letter grade, then check whether your state has posted a current algae or bacteria advisory for that specific water before you get in.

Each lake is matched against the Army Corps of Engineers National Inventory of Dams. A lake with a co-located dam record is treated as a reservoir; a lake the inventory was searched for and returned nothing on is treated as natural. Lakes that have not been through that match are left out of both columns rather than assumed natural. 2,415 lakes carry a dam match, 1,529 of them with a full A–F grade across 33 states.

No. Reservoirs are built where rivers are, which is disproportionately in farmed and settled watersheds, so land use and impoundment are tangled together in this table. What the numbers support is the practical point: the average impounded water body carries a heavier nutrient load than the average natural one, and the letter grade on the individual lake is what settles the individual case.