Is Rock Snot Affecting Your Rating Curve?

hydrological data, hydrological methods, hydrology, hydrology blog, hydrology corner, water data, Rating Curves, Monitoring

Didymosphenia geminata is a freshwater alga that forms extensive blooms on rocky stream beds. The common name for Didymo is ‘Rock Snot’. The common name is very descriptive of small quantities but I think that extensive blooms look more like a raw sewage spill. Long plumes of the brown mats will bleach white giving the appearance of strands of toilet paper trailing from a septic sludge.

Didymo is a northern circumpolar species preferring cold, clear water. It is spreading rapidly with the primary mode of dispersion suspected to be contaminated fishing/wading gear. It is a problematic invasive species because it’s effect on the food web of a stream starting with the displacement of common macro invertebrates and native alga.

hydrological data, hydrological methods, hydrology, hydrology blog, hydrology corner, water data, Rating Curves, MonitoringAttempts to control the spread of Didymo include banning felt soled wading boots. Some retailers have already eliminated felt-soled boots from their product line.  Felt soles are preferred by many stream hydrographers to provide the traction needed to measure fast flowing streams. Bans such as already exist in Alaska, Vermont and New Zealand will affect the safety of wading measurements at many locations. Felt soled waders are most effective where you have a smooth cobble bed with a thin biofilm of slippery green alga. You do not want to be responsible for introducing Didymo to these streams but you also don’t want to suffer injury as a result of a fall on the slippery rocks. For these locations you may want to cache a pair of felt soled waders in the gauge house to be used exclusively at that location.

Blooms of Didymo will affect a rating curve

The masses can be 10 to 20 cm thick with strands trailing for up to a meter in the current. Didymo is a silica-based diatom that creates a polysaccharide and protein stalk to attach to submerged rocks. These stalks are produced prolifically and are highly resistant to degradation. The thickness of the mat will affect the rating curve offset, which means the effect increases as stage decreases requiring a stage-based shift to the curve. An invasion of Didymo at a previously pristine gauge location will require an increase in measurement frequency, particularly at low-flow, to characterize the shape of the shift corrections as the mats form and decay.

Didymo will rapidly foul any in-stream sensor. It forms a heavy felt-like mat that completely covers every surface. Much more frequent cleaning of the sensors will be required. I do not know how an Acoustic Doppler Velocity Meter would be affected but I can’t imagine it would be good for acoustic fidelity. Bubblers may not be affected as badly but would also require more frequent inspection and maintenance.

Prevention Tips

To help reduce the spread of Didymo always check for evidence of contamination, clean with hot water and bleach or detergent and thoroughly dry any wading or metering gear that has been in contact with the water.


Stu Hamilton

Stu Hamilton

Senior Hydrologist

Hydrology field work done today, if managed well, becomes part of a legacy of information that will serve for generations to come. As an avid canoeist and whitewater kayaker I was easily drawn into a career in hydrometry in spite of an undergraduate education in biology. Shortly after graduating from the University of Alaska I started work with the Water Survey of Canada in Whitehorse, Yukon. The initial appeal was the freedom to travel extensively to some of the most beautiful landscapes on the planet to measure streamflow. The highlight of my career was measuring 7040 m3s-1 of flow on the Porcupine River using a small, under-powered, aluminum skiff, kevlar tagline and a 150 pound sounding weight. It took 4 tries to string the line, while uprooted trees and large ice floes came down the river. I am guilty of being a data philosopher. I think we have to first be able to clearly articulate what an ideal data set should look like and then we can influence the direction of technological development to make that ideal achievable.

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