Thick didymo mats resemble fiber-glass insulation or wet toilet paper, inspiring its nickname, “rock snot.” It is generally light tan to brown in color (not green), with stalks sometimes forming long white strands. Clumps of didymo are not slimy, resemble wet wool, and are tough to pull apart.

Where is didymo found?

Native to northern North America and Europe, didymo has rapidly expanded its range, invading streams in several western states before moving east. Didymo has been found in several major New York water-based recreational rivers.

Where can rock snot be found?

California: Rock snot has been around in Northern California for quite some time, and is commonly found on the North and South Fork of the Yuba River. It can also be found in reservoirs such as Lake Shasta, Bullard's Bar Reservoir, and Scotts Flat Lake.