Using a Secchi disk is one of the simplest and oldest standardized methods for measuring water clarity. The technique involves lowering a circular disk into the water until you can no longer see it, then recording that depth. First described by Pietro Angelo Secchi in 1865, the method has remained virtually unchanged because it works remarkably well for a tool that costs almost nothing and requires no batteries or calibration.1J. Eur. Opt. Soc.-Rapid Publ. On the history of the Secchi disc Despite that simplicity, getting reliable readings depends on following a consistent protocol and understanding what the numbers actually mean.
What You Need and How to Set Up
A standard Secchi disk is a flat, round plate about 20 centimeters (roughly 8 inches) in diameter, painted in alternating black and white quadrants. It hangs horizontally from a calibrated line with a weight underneath to keep it from drifting sideways. The line is marked in increments, usually every half meter or every foot, so you can read the depth without measuring separately. You can buy ready-made versions from limnological supply companies, or make one from a dinner-plate-sized piece of marine plywood or acrylic, painted with waterproof paint. Some citizen science programs now use 3D-printed miniature versions that are surprisingly effective for inland water monitoring.2Frontiers in Water. Citizen Scientists Contribute to Real-Time Monitoring of Lake Water Quality Using 3D Printed Mini Secchi Disks
Beyond the disk itself, the only equipment you really need is a pair of polarized sunglasses (to cut surface glare), a notebook or waterproof data sheet, and a shaded spot on the boat or dock from which to lower the disk. If you’re working from a canoe or kayak, you’ll want the shady side of the vessel facing the sun so your own shadow falls over the water where you’re looking. A viewing tube (essentially a bucket with a clear bottom) can help eliminate surface glare in choppy conditions, though polarized glasses handle this well enough on calm days.
The Step-by-Step Measurement
The procedure is straightforward, but the details matter for repeatability. Lower the disk slowly on its line into the water on the shaded side of the boat. Watch the disk as it descends. At some depth, it will fade and vanish from view. Note that depth on the line. Then slowly pull the disk back up until it just reappears. Note that depth too. Your Secchi depth is the average of those two numbers: the depth where the disk disappeared going down and the depth where it reappeared coming up. Averaging the two readings reduces the influence of momentary changes in your focus or passing waves.
Aim to take your reading around solar noon, or at least between mid-morning and mid-afternoon when the sun is relatively high. Low sun angles send light through more water at a steep slant, which changes how deep you can see. Consistency in timing matters more than perfection: if you always measure at roughly the same time of day, your readings will be comparable to each other even if the absolute depth is slightly different from what you’d get at high noon.
Record the depth in meters (or feet, if that’s the convention for your monitoring program), along with the date, time, weather conditions, wind speed, and your location. If the water is so clear that the disk is still visible when it hits the bottom, record the bottom depth and note that the true Secchi depth exceeds it. If the water is so murky that the disk vanishes within the first few centimeters, record whatever depth you get but understand you’re approaching the limits of what the tool can tell you.
Why the Disk Disappears
The disk vanishes because the water between you and the disk absorbs and scatters the light bouncing off the white surface. In classical theory, sunlight hits the disk, reflects back toward your eye in a more or less direct beam, and that beam gets progressively dimmer as it passes through water containing dissolved and suspended particles. The disk becomes invisible when the contrast between reflected light and the surrounding water drops below what your eye can detect.
That model works well in moderately clear water, but research has shown it breaks down in very turbid conditions. In the murkiest waters, the disk can actually remain visible at depths up to four times deeper than the classical theory predicts. The explanation is that in highly turbid water, scattered photons still reach the observer’s eye through diffuse pathways rather than a direct beam, making the disk appear blurry but still detectable.3Journal of Geophysical Research: Oceans. Secchi Disk Measurements in Turbid Water This is worth knowing because it means that in extremely murky water, Secchi readings can overestimate clarity relative to what electronic instruments would measure.
Interestingly, the human eye-brain system doesn’t just passively record how bright the disk is. Experiments with color-filtered glasses have shown that in blue ocean water, observers can see the disk just as deeply through blue-pass glasses as with naked eyes, but much less deeply through green-pass glasses, even though green is closer to the peak sensitivity of the human eye. The brain appears to use contrast information in whatever wavelength band penetrates deepest, not just raw brightness.4PubMed. Secchi disk observation with spectral-selective glasses in blue and green waters In practical terms, this means the measurement is more robust than you’d expect from a purely optical standpoint: your visual system is already optimizing for the clearest signal.
Common Mistakes That Skew Your Readings
The biggest source of error is surface glare. If sunlight is reflecting off the water surface into your eyes, you’ll lose sight of the disk sooner than you should, and your reading will be artificially shallow. Always lower the disk on the shaded side of whatever platform you’re standing on, wear polarized sunglasses, and avoid measuring when the sun is directly in front of you and low on the horizon.
Another common mistake is lowering the disk too fast. If you drop it quickly, you’ll overshoot the disappearance point and end up with a depth that’s too deep. A slow, steady descent gives your eyes time to track the fading contrast. Likewise, pulling it up too quickly on the return will make you see it “reappear” later than it should, again biasing the reading deeper.
Boat drift is a subtler problem. In wind or current, the boat moves while the disk hangs below, so the line isn’t vertical. A line at an angle means the disk is shallower than the length of line you’ve let out suggests. In strong current, a heavier weight on the disk and a thinner line help keep things vertical. If you’re in a spot with significant drift, take the reading quickly and note the conditions.
Finally, there’s the observer-to-observer problem. Different people have different visual acuity, especially in the blue-green range that matters most underwater. Two observers measuring at the same spot and time can get readings that differ by 10 to 20 percent. For monitoring programs, this is handled by having the same person take readings at a given site over time, or by training multiple observers and comparing their results until they converge.
What Secchi Depth Tells You About a Lake or Coastal Water
A single Secchi reading is a snapshot of water transparency at one place and time. What makes the measurement ecologically useful is the relationship between transparency and the factors that reduce it: algae, suspended sediment, and dissolved organic matter (the brownish tint from decomposing plant material). In lakes dominated by algal growth, Secchi depth tracks algal abundance closely. In rivers or reservoirs with heavy sediment loads, it tracks suspended particles. The trick is knowing which factor dominates in your water body, because the management response is different.
One of the most widely used applications is estimating the euphotic depth, the layer of water that receives enough sunlight for photosynthesis. A common rule of thumb puts the euphotic depth at roughly 1.7 to 3 times the Secchi depth, though the exact conversion factor varies with the optical character of the water.5ISPRS International Journal of Geo-Information. Optimizing the Use of Secchi Depth as a Proxy for Euphotic Depth in Coastal Waters: An Empirical Study from the Baltic Sea In a coastal setting where dissolved organic matter shifts the color spectrum, that conversion can be quite different from a clear alpine lake. The point is that Secchi readings let you estimate where plants and algae can grow without needing an expensive light meter.
The Trophic State Index
Secchi depth is one of the three standard inputs to the Carlson Trophic State Index, a scoring system developed in the late 1970s that rates lakes on a scale from 0 to 100. Each ten-point jump on the scale corresponds roughly to a doubling in algal biomass.6Limnology and Oceanography. A trophic state index for lakes The other two inputs are chlorophyll-a concentration (a proxy for how much algae is present) and total phosphorus (the nutrient that most often drives algal growth in freshwater). You can calculate a trophic state index from any one of the three, but using all three together gives a fuller picture.
A TSI based on Secchi depth alone does have blind spots. In reservoirs where non-algal particles dominate the turbidity, like those affected by clay runoff or construction sediment, the Secchi-based score may flag a lake as eutrophic (nutrient-rich and algae-heavy) even though algal growth is modest. Research on drinking-water reservoirs in Taiwan found that when small non-algal particles dominate the water, the Secchi-based trophic score flagged eutrophication about 63 percent of the time, while the chlorophyll-based score, which actually tracks algal abundance, only flagged it 20 percent of the time.7Sustainable Environment Research. Eutrophication factor analysis using Carlson trophic state index (CTSI) towards non-algal impact reservoirs in Taiwan That’s a large disconnect, and it’s a reminder that a Secchi disk measures clarity, not algae. In sediment-heavy waters, pairing it with at least a chlorophyll measurement is important for interpreting the score correctly.
When the Secchi Disk Stops Being Useful
Every tool has a range where it works best, and the Secchi disk is no exception. In a comparative study of transparency and turbidity in a highly eutrophic reservoir, researchers found that when turbidity was below about 20 NTU, the Secchi disk clearly differentiated between water conditions. Once turbidity exceeded 40 NTU, the disk essentially bottomed out: readings were all very shallow and could no longer distinguish meaningfully between different levels of murkiness.8Water. Secchi Disk Depth or Turbidity, Which Is Better for Assessing Environmental Quality in Eutrophic Waters? A Case Study in a Shallow Hypereutrophic Reservoir At that point, an electronic turbidity meter becomes the better tool because it can still resolve differences the human eye cannot.
At the other extreme, in exceptionally clear open-ocean water, the disk may remain visible to 40 meters or more, and readings become sensitive to small changes in lighting, wave action, and observer fatigue. In these conditions, electronic profiling instruments and satellite-derived estimates provide more reproducible measurements. For the vast majority of lakes, reservoirs, estuaries, and coastal waters, however, the Secchi disk sits comfortably in its sweet spot.
Building a Long-Term Record
The real power of Secchi measurements comes from repetition over time. A single reading tells you the clarity of a lake on one afternoon. Monthly readings over a year reveal the seasonal cycle: spring turnover, the summer algal bloom, the autumn clearing, and winter ice cover. A decade of readings reveals whether a lake is getting clearer (often a sign that phosphorus controls are working) or more turbid (a sign of increasing nutrient loading, land-use change, or invasive species). Satellite-based methods have been used to assess water clarity trends across North America’s largest lakes over a 25-year period from 1998 to 2023, capturing shifts in seasonal and inter-decadal patterns.9Journal of Great Lakes Research. Assessing water clarity status and long-term trends in North America’s largest lakes using ESA’s Ocean Colour Climate Change Initiative (OC-CCI) products But those satellite records need ground-truthing, and the ground truth, in many cases, is still someone lowering a painted disk on a string.
This is where citizen science programs have found a natural home. Because the method is cheap and requires no specialized training beyond a brief orientation, volunteer lake monitors around the world have been generating long-term datasets that no professional agency could afford to collect on its own. Programs like the Secchi Dip-In in North America coordinate thousands of volunteers to take simultaneous readings on a single day each summer, creating a snapshot of continental-scale water clarity. Miniaturized 3D-printed disks have extended this model by making the equipment even more accessible and consistent across volunteers.2Frontiers in Water. Citizen Scientists Contribute to Real-Time Monitoring of Lake Water Quality Using 3D Printed Mini Secchi Disks
If you’re starting your own monitoring program, the most important thing is consistency. Use the same disk, the same observation protocol, the same general time of day, and ideally the same person taking readings at each site. Record everything, including conditions that seem irrelevant at the time, like whether it rained the day before or whether the lake was unusually calm. Those notes become invaluable when you’re trying to interpret an anomalous reading years later.
Water Clarity and Property Values
Secchi depth readings have found an unexpected second life in real-estate economics. Researchers studying the relationship between water quality and housing prices in the United States have used water clarity, often measured by Secchi depth, as a key variable. A comprehensive meta-analysis pooling data from 36 hedonic pricing studies across the country found that water clarity consistently influences how much people will pay for lakefront or waterfront homes.10PubMed Central. Property values, water quality, and benefit transfer: A nationwide meta-analysis The clearer the water, the higher the premium. This makes intuitive sense: people want to swim, fish, and look at appealing water, not green soup. But the formalization of that relationship means Secchi readings now feed into cost-benefit analyses for lake-restoration projects. A municipal government deciding whether to spend money on stormwater controls or phosphorus removal can point to Secchi depth improvements and estimate the dollar value of the resulting bump in property assessments.
This economic dimension has quietly pushed water clarity into regulatory and planning conversations that used to be driven entirely by chemical parameters like nutrient concentrations and dissolved oxygen. A Secchi reading is something a town council member can understand on sight: “the lake went from 1.5 meters to 3 meters” communicates a change that “total phosphorus declined from 40 to 25 micrograms per liter” does not, even though they may be describing the same improvement.
Making Your Own Disk
You don’t need a commercial Secchi disk to get started. A 20-centimeter circle of exterior plywood or white acrylic sheeting works fine. If you’re painting quadrants, use flat (not glossy) marine paint: one pair of opposite quadrants white, the other pair black. Drill a hole in the center and bolt an eyelet through it so you can attach a line above and a small weight below. A fishing sinker or a few heavy washers will keep the disk horizontal as it descends.
For the line, braided nylon or polyester cord is better than monofilament, which is harder to mark. Use a permanent marker or small cable ties at measured intervals. Check your markings against a tape measure before you start. Over time, some line materials stretch, so recalibrate at the start of each monitoring season.
Disk size matters somewhat. The standard 20-centimeter disk is designed for open water viewed from above. If you’re monitoring a narrow stream or a small pond where you can only get close to the surface, a smaller disk (8 to 12 centimeters) may be more practical, though your readings won’t be directly comparable to those made with a full-size disk. For ocean use, some protocols call for a 30-centimeter all-white disk. The key is to use the same disk every time at a given site.
Handling Tricky Conditions
Windy days create surface chop that breaks up your view of the disk. If you can’t postpone the measurement, a simple viewing tube helps. Take a bucket, cut the bottom out, and replace it with a piece of clear acrylic or glass sealed with waterproof caulk. Press the open end into the water surface and look through the clear bottom. The tube eliminates surface reflections and wave distortion. Some monitoring programs carry a dedicated viewing scope, but a bucket works in a pinch.
Colored water is another wrinkle. Lakes stained brown by tannins and humic acids from surrounding wetlands can have low Secchi depths not because of algae or sediment, but because the dissolved organic matter absorbs light. In these “brown-water” lakes, you might get a Secchi reading of 1.5 meters in water that has almost no algae and very little sediment. The reading is still a valid measure of clarity and light penetration, but it doesn’t mean the lake is polluted or eutrophic. This is one of the reasons the Carlson index works best when you compare the Secchi-based score to the chlorophyll-based score: if the two diverge sharply, something other than algae is controlling the transparency.
Ice-covered lakes present an obvious problem: you can’t lower a disk through solid ice. Some winter monitoring programs drill an auger hole and take a reading through it, but the ice hole creates odd lighting conditions (the surrounding ice blocks ambient light from reaching the water column), and results can differ from open-water readings. If you’re doing year-round monitoring, note the method change and treat winter readings as a separate dataset.
Secchi Disk Readings in Flowing Water
Most Secchi disk protocols are designed for still or slow-moving water: lakes, reservoirs, and calm coastal bays. In rivers and streams, the current can push the disk sideways and tilt the line away from vertical, making depth readings unreliable. Suspended sediment loads also change rapidly with flow, so a reading taken during a rain event may bear little resemblance to one taken during baseflow conditions a day later.
For stream monitoring, a horizontal version of the idea is more common: a Secchi tube, sometimes called a transparency tube, is a clear plastic column roughly 60 to 120 centimeters long with a small Secchi-patterned disk fixed at the bottom. You fill the tube with a water sample and look down through it, draining water from a valve at the bottom until the disk pattern becomes visible. The depth of water remaining in the tube gives you a transparency reading in centimeters. It’s not a true Secchi depth in the limnological sense, but it measures the same property (visual clarity) and is well suited to wadeable streams where lowering a disk from a boat makes no sense. Many state water-quality monitoring programs in the U.S. use transparency tubes for their volunteer stream-monitoring networks.
The tube method has its own quirks. Because the water sample is held still, you’re measuring the clarity of a grab sample, not the in-situ water column. Particles that would normally stay suspended in flowing water may settle during the measurement. For best results, take the reading immediately after filling the tube and before settling has time to change the sample.
Satellite Estimates and Ground Truth
Satellite remote sensing can estimate Secchi depth across entire lakes or ocean regions from space, using the color and brightness of water detected by orbiting sensors. These estimates are enormously valuable for monitoring large or remote water bodies where nobody is regularly lowering a disk. Studies have tracked clarity trends across large North American lakes over multiple decades using satellite ocean-color data.9Journal of Great Lakes Research. Assessing water clarity status and long-term trends in North America’s largest lakes using ESA’s Ocean Colour Climate Change Initiative (OC-CCI) products
But satellite-derived Secchi estimates depend on calibration against real-world measurements, and those ground-truth measurements are usually made with the original low-tech disk. Every satellite algorithm has to be trained and validated using actual in-water observations, and whenever a new sensor is launched or an algorithm is updated, the community goes back to its archive of disk readings. In that sense, the painted disk isn’t being replaced by technology so much as being amplified by it. The volunteers and technicians taking readings at specific coordinates on specific dates are generating the calibration points that make continental-scale satellite monitoring possible.