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Cover types, as referred to in this report, represent differences in the spatial relation of emergent cover to open water or exposed bottom soil. Emergent plants used to describe cover types include species of low-prairie and wet-meadow zones in ephemeral (Class I) and temporary (Class II) ponds, but are restricted to species of shallow-marsh and deep-marsh zones in seasonal (Class III), semipermanent (Class IV), permanent (Class V), and alkali (Class VI) ponds and lakes. In cultivated ephemeral (Class I), temporary (Class II), and seasonal (Class III) ponds, cover may also be represented by planted crops including small grain, row crops, and forage species used for hay. Cover in fen (Class VII) ponds is represented by typical emergent fen species. Cover interspersion in natural ponds and lakes can be one of four types:

Cover type 1.

Closed stands of emergents with open water or bare soil covering less than 5 percent of the wetland area.

Cover type 2.

Open water or bare soil covering 5 to 95 percent of the wetland area, with scattered dense patches or diffuse open stands of emergent cover. Closed stands of emergents, located in the central portion of a pond or lake and surrounded by open water along the shallow margins, are included in this cover type.

Cover type 3.

Central expanses of open water or bare soil (comprising more than 5 percent of the wetland area) surrounded by peripheral bands of emergent cover averaging 6 feet or more in width.

Cover type 4.

Open water or bare soil covers more than 95 percent of the wetland area. This cover type also includes small ponds in which emergent cover is restricted to marginal bands less than 6 feet in average width.

The basic cover types, showing common variations, are illustrated in figure 3.

Figure 3. Basic cover types of natural ponds and lakes showing common variations in aspect. White areas indicate open water or exposed bare soil; shaded areas indicate emergent vegetation.

All cover types are represented in ephemeral (Class I), temporary (Class II), seasonal (Class III), and semipermanent (Class IV) ponds or lakes, although their frequency of occurrence varies considerably. Except for brief periods in the spring, cover type I is prevalent in undisturbed ponds or lakes of Classes I to III, while cover types 2 and 3 are frequent in Class IV. Cover types 2, 3, and 4 are quite common in cropland ponds or lakes of Classes I to III that have been disturbed by intensive cultivation. Only two cover types (3 and 4) are represented in permanent (Class V) and alkali (Class VI) ponds and lakes. Fen ponds (Class VII) include cover types 1, 2, and 3. Cover type variations are illustrated in plates 1 to 31.

Changes in cover type occur frequently in many ponds and lakes in this region owing in large part to variable climatic conditions and resulting fluctuations in water levels. Alternations in the type and intensity of land use may occasionally be important causes. Other phenomena sometimes responsible for changes in cover type include "eat-outs" of emergent vegetation by muskrats, uprooting of emergent plants by flotation of bottom ice in the early spring, thinning of emergent growth by heavy siltation of ponds or by scouring of bottom soils by severe wave action, and poor development of new emergent growth because of excessive matting of dead vegetation.