Monmouth County Lake Quality
New Jersey, 4 lakes, average grade C (Fair)
Only 4 lakes in Monmouth County carry an active grade. The county-level summary is therefore representative of those specific lakes, not necessarily of every water body in the county. Monmouth County averages to a C. The distribution behind that average matters more than the letter: a few stressed lakes versus many decent ones can produce the same average as the inverse.
Most of the county's monitored lakes are shallow — under 50 feet — which means wind keeps the water column mixed all summer and sediment-bound phosphorus releases back into the surface layer whenever the bottom is disturbed. Lake Topanemus (B) is the cleanest in the county.
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Quick Answers for Monmouth County
Planning a trip? Check special fishing regulations for Lake Topanemus — or check the official New Jersey fishing regulations.
All Lakes in Monmouth County
4 lakes · 3 graded, 1 not rated3 graded lakes · 1 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | BLake TopanemusMonmouth | B | 5.4 ft | 24.9 µg/L | 10.2 µg/L | No recent samples | Not recorded |
| 2 | CSwimming River ReservoirMonmouth | C | 4.8 ft | 28.4 µg/L | 23.4 µg/L | No recent samples | Not recorded |
| 3 | FSylvan LakeMonmouth | F | 1.8 ft | 82.9 µg/L | 31.6 µg/L | No recent samples | Not recorded |
| NRDeal LakeMonmouth | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded |
NR = not rated: the monitoring record does not meet the evidence bar for a grade. That is EPA Integrated Reporting Category 3 (insufficient information), not a failing grade. How grading works.
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Other Counties in New Jersey
See how Monmouth County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 49 states and 2 Canadian provinces.
Source: EPA National Aquatic Resource Surveys, 2026.