Potamogeton crispus
Leaves lanceolate to linear-lanceolate, usually 3–6 cm long, with serrate and usually undulate or crisped margins, 3–5-veined, without faint intermediate veins; ligule obtuse. Fruit with a beak c. as long as the fruit. Spikes c. 1 cm long, few-flowered. Widespread. Fl. spring–summer