In his book Fly Fishing: Some New Arts and Mysteries (1914), Englishman J.C. Mottram devoted a chapter to aquatic plants. Here’s an excerpt:
“Here an attempt is made to analyse water weeds, to separate the good from the indifferent, the indifferent from the bad. The number of weeds that green our streams is not legion. Many can be dealt with in groups, so the task is not very formidable. The relatively useless weeds are first described, then the more useful, and last the most useful. Elodea canadensis, the Canadian water weed, heads the list, for not only is it useless, but positively harmful.”
Elodea is native to North America, invasive to Europe. Today it is still considered harmful in the British Isles and on the Continent, and efforts to control and/or remove it are widespread. Elodea has a broad range across the United States, and depending on the situation can be considered a nuisance species here. I find it frequently on the streams and lakes that I spend most of my time on, but not in great quantity.