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01 · The Fetch Principle
Fetch — the actual word — is the uninterrupted distance wind travels across open water before reaching a given point. It's a foundational concept in physical oceanography, and it's the single biggest variable that determines water surface conditions on any lake.
A north wind at 12 mph does completely different things to different parts of the same lake. The southwest cove with 400 meters of land-buffered fetch? Glass. The main channel with 3,200 meters of open exposure directly into that same wind? Whitecaps building. Same wind. Same lake. Completely opposite conditions.
Standard weather apps give you one number for the whole lake. That number is not wrong — it's just answering the wrong question. We answer: where, specifically, is the water clean right now?
Stylized bird's-eye view — same wind, different conditions by fetch exposure
02 · The Formula
Wind speed doesn't scale wave energy evenly — it squares. Double the wind speed and you get four times the wave-building energy at a given point. That relationship is captured in the core exposure equation we apply to every grid point on every lake.
Fetch distance is calculated geometrically for each point on the lake — measuring the unobstructed water surface in the upwind direction against the real shoreline polygon. The result is a single energy number that powers the color classification.
Notice: same wind speed across all three scenarios. The variable is purely the geometry of the lake. That's why a single weather station can never tell you what you actually need to know.
03 · The Grid
Morse Reservoir is covered by a 30-meter grid — one data point every 30 meters across the entire lake surface. Each point gets its own fetch calculation, depth lookup, and condition classification on every single request.
30 meters is the resolution of real bathymetric data. It's fine enough to distinguish the inside of a cove from the point of land next to it. That precision matters — a 60-meter grid would miss the pockets of glass that are exactly what you're driving out to find.
Every point is independently scored. There's no interpolation, no blending, no "this region is probably similar to that one." The geometry is run fresh on each call against the live wind reading.
Stylized 30m grid — each dot = one scored point
04 · Depth Context
Wave energy interacts with the lake floor. In shallow water — anything under about 8 feet — the bottom friction amplifies surface chop and causes waves to build steeper and break earlier. The same energy hitting a 20-foot channel produces rolling swells. The same energy in a 3-foot flat produces short, messy chop that's rough to ride in.
We layer in USGS bathymetric depth data at each grid point. The depth at each location modifies the exposure score with a multiplier: deep points get a slight reduction (cleaner than raw fetch suggests), shallow points get a bump (worse). This brings the map's predictions in line with what you actually find on the water.
The depth adjustment is subtle — it's not the primary variable, fetch distance is — but it's the difference between a model that's 80% accurate and one that actually matches what you find at the ramp.
Lake cross-section · depth × wave behavior
05 · The Scoring System
Every scored grid point falls into one of five condition tiers based on its exposure score. The overall session score (1–10) is derived from the weighted distribution of points across these tiers — heavily penalizing whitecap percentage and rewarding glass percentage.
The percentage of the lake in each tier is shown in the sidebar. When 60%+ of the lake is glass-level, you're looking at a send-it morning. When 40%+ is rough or worse — it's showing you where to avoid, not where to go.
06 · Sport-Specific Thresholds
A barefoot skier needs near-perfect glass to stay on their feet. A wakeboarder can handle moderate chop and still throw tricks. A wake surfer sitting 10 feet behind a 100,000-pound boat creates its own wave — the surrounding surface matters less than the swell coming off the hull. These differences are real, and they're tuned into the scoring system.
Sport mode adjusts the score calculation by shifting the tier boundaries — lowering what counts as "acceptable" for high-sensitivity sports, raising the ceiling for low-sensitivity ones. The same conditions score an 8.2 for wakeboarding and a 5.4 for barefoot. That's not a bug. That's the actual difference in what the water looks like from each rider's position.
07 · The Bottom Line
Three things happen when you use Fetch instead of checking the wind and guessing.
You know how it works. Now see it live.
Live on Morse Reservoir. Geist coming next. Free during beta.