Sparganium angustifolium is mostly restricted to the northern third of Wisconsin (see map), but within that area it is very common in lakes. The plants are rooted in the lakebed and the highly flexible leaves float on the water's surface. This species is usually easy to recognize, because of the very narrow floating leaves, less than 5 mm wide (often much less). The inflorescence is unbranched, there are usually 2 or more male flower heads, and mature fruiting heads are usually at least 15 mm in diameter. Sparganium fluctuans also has floating leaves, but they are wider (> 5 mm) and the inflorescence is clearly branched. S. natans (= S. minimum) has similar narrow floating leaves, but the inflorescence typically has only one male head and the mature fruiting heads are less than 12 mm in diameter. S. chlorocarpum is ordinarily erect, but it may produce floating leaves under extreme conditions, especially in rapidly flowing water and such plants usually do not flower. S. angustifolium is not found in rapidly flowing water.

It is widely acknowledged that Sparganium plants can be very difficult to identify under some circumstances. Individual plants can vary on just about any character, so it is essential to take good notes on the population and, if a voucher is taken, to collect sufficient material to reflect the general condition of the population.