Read the water
Water clarity
How far light gets down decides color, presentation, and depth. Here's how to actually know it — and set it for the plan.
Drop a light lure — where does it vanish?
Water color
How to really know
Two honest moves: look it up before you go, then confirm on the day. A baseline can be decades old; a thunderstorm last night or 20 mph today blows it out. Clarity also varies by spot — clear main lake, muddy windblown bay, stained river arm. Read it per area, not per lake.
The one tool to build — a Secchi tube (~$15)
A clear ~1.5" tube, 60–120 cm, with a black-and-white disk at the bottom and a cm scale. Fill with lake water, look straight down, drain until the disk reappears, read the depth. It's the 160-year-old gold standard — cheaper, more reliable, and more interpretable than any electronic option. Drop it in every trip.
What the numbers mean
| Secchi | Look | Bucket |
|---|---|---|
| > 13 ft | Gin-clear (oligotrophic) | Clear |
| 6.5–13 ft | Greenish, see a few feet | Clear → Stained |
| 1.5–6.5 ft | Murky, algae or stain | Stained |
| < 1.5 ft | Can't see past a foot | Muddy |
Sensors — the honest ranking
DIY Secchi tube — best ROI, no calibration. Online lookup — great pre-trip baseline (see below).
Arduino turbidity sensor (TS-300B/SEN0189, ~$10) — a fun build but qualitative only, with calibration and fouling headaches. Good for a fixed dock logger, not trip-to-trip.
The Garmin does not measure clarity.Sonar/CHIRP "clarity" means image sharpness, not water cloudiness — there's no turbidity value to read. (It does give great surface temp.)
Look up your lake (Minnesota)
For most MN lakes, clarity is already measured — often decades of it:
- MN DNR LakeFinder — per-lake water quality & clarity.
- UMN LakeBrowser — satellite Secchi for 10,500+ lakes, 1985–present.
- EPA Water Quality Portal — machine-readable Secchi history.
A future build can pull these to seed a per-lake baseline automatically.
Methods, sensors, and sources: docs/research/06-water-clarity.md.