Water clarity is a critical issue to fishermen, hydrologists, water quality experts, limnologists, homeowners and public health and utilities officials. Clearly (pun intended), the meaning of transparent water varies among those who use the term to describe any particular source of water. Let’s examine what it means to trout fishers.
Natural water streamflow in our creeks and rivers is the combined result of springs (groundwater), surface runoff, and atmospheric deposition. As these sources of water combine and flow, the fluid scouring on the streambanks and riverbed may suspend particles of sediment previously deposited in the stream, thereby causing apparent “cloudiness” in the water. This can range from a very slight opaqueness to a virtual mudpie flowing toward the ocean. The higher sediment loads, because they are heavier than pure water, can only be carried if the flow rate and turbulence are high enough to keep the sediment particles in suspension. Hence, the water “muddiest” in appearance is seen most often during flood events created by thunderstorms, hurricanes, or catastrophic failure of dams or other water control structures. Contrast this kind of runoff with the gentle percolation of snowmelt in the spring or gentle summer rains.
The fisher, of course, will be primarily concerned about whether or not the fish can find their bait. Since trout use a combination of sound, olfaction and, primarily, sight to locate prey, the transparency of the water will determine the field of vision available to them at any time. The question now becomes “how do we measure transparency of the water” as an indication of what kind of lure to use to catch trout.
The simplest device for this task is a transparency tube, a clear plastic cylinder about a meter in length and 5 cm in diameter. Along the length of the cylinder is a tape or painted measurement scale calibrated in centimeters. Resting at the bottom of the tube is a secchi disk, a target divided into 4 quadrants of alternating black and white “pie” wedges. The disk is attached to a string that emerges from the top of the cylinder so that the disk can be raised or lowered in the cylinder.
To measure the transparency of a sample of water, simply hold the cylinder in a vertical position and fill the tube with the sample of water. Use the string to pull the secchi disk toward the top of the tube while looking into the cylinder. When the target disk is just visible, record its height in the tube by reading the number from the meter tape on the tube. Now lower the disk until the target is no longer discernable, and record the number from the meter tape. Add the numbers together and divide by two to get an estimate of the transparency of the water. The transparency range of this device is then from zero (can’t see the disk until it reaches the top of the tube), to 1 meter. Any reading greater than 90-100 cm would be considered “clear” water.
For an example, a water sample was collected from Camp Creek at the Fillmore St. bridge in Preston, Minnesota, on April 26, 2018. Repeated measurements with the transparency tube yielded an estimate of 24 cm. (about 10 inches). To the fisher’s eye, one would call the creek “cloudy” or “off-color”, but most would conclude that live bait or an attractor lure (spinner, plug, streamer) could still be fished with good chances of success. Also note that you will be able to “see the bottom” of the creek under normal daylight conditions to a depth of about 18-24 inches. We can expect a trout to “see” even further if the target is something flashy, noisy, or smelly. Thus, we would say the conditions are “cloudy, but fishable”.
Relating the transparency measure to stream flow is simply a matter of measuring each of these variables simultaneously at repeatable sites. Higher stream flows (measured as CFS or Cubic Feet per Second) would be expected to correspond to higher sediment burdens (from surface sediments in runoff and re-suspension of streambed sediments). There is a stream gauge at Carimona, upstream from Preston on the South Branch of the Root River. We can use it to establish a correlation between transparency at Preston and streamflow at Carimona, thus providing a measurable index of how recent rainfall or runoff events may have altered water clarity downstream of the measurement sites.
As an example, we estimated a 42 cm transparency of the South Branch of the Root River on April 26, 2018, at the County Rd. 17 bridge in Preston. Comparing this with a stream flow estimate at the Carimona gauging station of 170 CFS, implies that fishing downstream of Carimona will occur in slightly cloudy to clear water if flow rates remain below 170 CFS, and if no additional sources of sediment are added into the river from downstream tributaries (Willow Creek, Camp Creek).
The National Trout Center will endeavor to establish an archive of transparency measurements, together with other stream conditions for the Root River valley over the course of the trout fishing season.