Lake Recreation Safety: What Assessment Data Tells You

What the Recreation Verdict Measures

Recreation is one of the designated uses states assess under the Clean Water Act, and the best-known standard behind it is built for primary contact: activities like swimming where people are likely to ingest water. States set their own numeric criteria, generally following the recommendations the U.S. Environmental Protection Agency published as its 2012 Recreational Water Quality Criteria. When monitoring exceeds them, the use is reported as Not Supporting.

What that leaves out, and what matters most for reading a lake page here, is that "recreation" is not one question. States file their own use names, and EPA groups them together: this dataset contains 42 distinct recreation use names, ranging from "Primary Contact Recreation" and "The Use of Waters for Swimming and Other Primary Contact Recreation" through "Secondary Contact Recreation" and "Boating" to broad labels such as "Recreation" and "Public Health and Welfare" that state no contact level at all. A lake page shows one rolled-up verdict, and PlainLakes rolls it up conservatively: if the state ruled any recreation use Not Supporting, the verdict is Not Supporting.

So the verdict is not automatically a statement about swimming. Of the 3,636 lakes in this database carrying a recreation impairment, 2,073 (57%) have a use whose name states swimming-level contact actually ruled Not Supporting. 96 fail only on a secondary or non-contact use such as boating or wading, which is held to a lower bacteria standard, and 1,467 fail only on a use whose name does not say which. The same applies in reverse: of 6,116 lakes reported Fully Supporting, 379 pass solely on a secondary or non-contact use, meaning the state did not say that swimming meets its standard there. The use table on each lake page names the individual uses and their verdicts, and that table, not the rollup, is what tells you which question the state answered.

The verdict is therefore a statement about the past: during the assessment cycle, the evidence the state used either stayed within its criteria or did not. That evidence is usually monitoring at monitored locations, and where the state filed the basis the lake page names it. The state also rates its own confidence in a verdict on some records, and where it filed that rating the lake page prints it beside the verdict. A state can file a finding as monitored and still record its confidence in it as Low. It aggregates over time, over the assessment unit, and over every recreation use the state assessed, which is exactly what makes it good background and poor real-time guidance. The cycle is when the state reported; the monitoring year, where filed, is when it measured. How verdicts and statuses work in general is covered in the assessment status guide.

Two Different Impairment Stories

Recreation impairment is popularly associated with fecal contamination, and on this dataset that is the smaller of the two stories. Counting only causes the state itself tied to a recreation use, 2,262 of the 3,636 impaired lakes (62%) carry a nutrient or algae-related cause, against 1,054 (29%) carrying a fecal indicator such as E. coli, fecal coliform or enterococcus. The two overlap on many lakes, and a remainder carry neither. The distinction is worth holding because the two behave differently in time, and the advice that follows from them is different.

Fecal indicators come from mundane, local sources: failing septic systems near shore, sanitary sewer overflows, stormwater carrying pet and wildlife waste, livestock runoff, and resident waterfowl flocks. Two properties matter for how you read the listing:

• It is episodic. Bacteria spike after rain washes the watershed into the lake, then decline in dry, sunny weather. Many impaired-for-recreation lakes are fine most dry days and risky after storms. A common rule of thumb from beach programs is to avoid swimming for a day or two after heavy rain, at any lake.
• It is fixable. Unlike mercury, pathogen sources are usually local infrastructure and land-use problems. Sewer repairs and septic enforcement have delisted waters, so a recreation impairment's first-listed cycle is worth checking: an old, persistent listing suggests an unresolved chronic source.

What the indicator itself measures, and the other causes it sits alongside, is explained in the impairment causes guide.

Harmful Algal Blooms: Context, Not Coverage

Harmful algal blooms, dense growths of cyanobacteria that can produce toxins, are a recreation hazard a biennial reporting cycle can only partly capture. Blooms form in days during warm, calm, nutrient-rich conditions and can dissipate as fast, so what an assessment mostly records is the underlying susceptibility, as the nutrient and algae listings above.

It captures more than susceptibility, though, and it is worth knowing where to look: 298 lakes here carry a cause naming a bloom or a bloom toxin outright, filed as "Harmful Algal Blooms", cyanobacterial microcystins, anatoxins or algal toxins. A listing of that kind is a state recording that the problem has actually occurred, not merely that conditions favour it. What it still cannot tell you is whether a bloom is present this week.

General points of context worth knowing: blooms often look like spilled paint, pea soup, or surface scum, but appearance is not a reliable toxicity test; toxin production varies even within one bloom; children and dogs are the usual concern in advisories because of their size and behavior in the water, and dogs have died after ingesting bloom material. When a bloom advisory is posted, treat it as governing. A lake's nutrient status and trophic class, covered in the trophic status guide, are the best assessment-data predictors of bloom susceptibility.

A Practical Routine for Using Both Layers

• Before a trip: read the lake's PlainLakes page for its recreation verdict, the individual uses behind it, its listed causes, and how recently it was assessed. A fecal-indicator listing tells you to be rain-aware; a nutrient or algae listing tells you to be bloom-aware in late summer; a verdict resting only on a boating use tells you the state has not ruled on swimming.
• Close to the day: check the county or state health department beach page and any state bloom advisory map for current postings.
• At the water: respect signage, skip water that looks like scum or spilled paint, and keep dogs out of anything questionable.
• After heavy rain: give any lake, listed or not, a day or two before swimming.

To scan conditions across a region before choosing a destination, the state pages summarize verdicts for every assessed lake in a state, and common questions about the data are answered in the site FAQ.