At our San Diego breaks, some days the bottom is clearly in view, and others, you’re lucky to see your feet. There are a lot of factors that drive this variability, and surprisingly, it’s often not the quality of water.
Seawater is close to transparent on its own. It’s the stuff in it that affects how clear it looks. A sand grain that gets split into a thousand pieces adds no mass to the water relative to its whole form. Each piece is a tenth of the original diameter, so the total surface has multiplied by ten, and all of that new surface is available to bounce light around. The particles are what you cannot see through, and it is their combined area in your line of sight that does it, not their mass.
Wavelength sets how far a wave’s orbital motion reaches down, and wavelength comes from period, so period is what decides whether a swell touches the bottom at all. A one meter wave in 20 meters of water moves the bed back and forth at about 11 cm/s when its period is 6 seconds, and 28 cm/s at 12 seconds.
Medium sand, around two tenths of a millimeter, needs roughly 25 cm/s at the bed under a 12-second wave before it lifts. In 20 meters of water, that’s about where a 1-meter swell of 10 seconds or longer sits. At 6 seconds, the same wave height only puts 11 cm/s on the bed, which moves nothing.
How long the sediment stays up depends mostly on grain size. A 200 micron sand grain falls through 10 meters of water in about 7 minutes, and 20 micron silt takes closer to 8 hours. That’s still-water math, though. Any turbulence in the water column keeps the fine material up far longer, which is why the murk outlasts the swell that made it by days.
After the sediment is suspended, it gets advected around. Speiser and colleagues put Sentinel-2 imagery over 31 sites in Northern California in 2025 and fit an exponential decay to turbidity heading offshore, and they found the turbid zone grew with wave energy and reached kilometers out during large events. Sandy beaches threw a lot of sediment into the water close to shore, but it faded quickly with distance. Rocky shores put in less and carried it much farther out, which is about what you’d expect from better organized rips running over uneven bottom.
Some of the murk shows up from below and has nothing to do with the swell. Internal bores run up the shelf on the tidal beat, scraping sediment off the slope as they go. Masunaga and colleagues tracked bores that displaced isotherms 20 meters vertically in 40 meters of water, with the strongest resuspension right at the head of the bore and the suspended material peeling off into a layer that spread offshore above the thermocline. That’s the structure you’re swimming through when the top few meters are warm and clear and everything below is cold and brown.
Biology also varies how much stuff is in the water. Upwelling brings nutrients toward the surface, a bloom follows within days to weeks, and it can cut visibility the same way sediment does. Runoff adds its own pulse of fine terrestrial material after a storm, and that’s the one case where bad visibility and bad water quality reliably show up together. The rest of the time they’re unrelated. Water you can see a long way through isn’t necessarily water you want in your sinuses, and water you can’t find your own hands in can be perfectly clean.
Further Reading:
What I do wonder about is - where does the science and technology of predicting visibility stand these days?