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53 Riparian ecosystems are severely end'Lngered in the Sacramento Valley of California where losses have been estiolated at 98.5 percent (I'/arner 1979). Much of the remaining linear mileage of riparian ecosystem is in varying stages of deterioration and subject to attack by habitat modification (see Chapter 18). Along the Sacramento River in California, croplands cover over 66 percent of the terrace lands that once supported riparian forests. Conversion has been close to 100 percent beyond the flood control levees. Riparian bottomland dominated by hard.woods in MJLssouri has declined by 96 percent. Along the lower Missouri River, at least 72,900 ha (180,000 acres) that were once part of a wide riparian floodplain have be"ome croplands as a result of channelization, bank stabiHzation activities and changed hydrologic regimes resulting from upstream io~oundment by dams and reservoirs for flood control and navigati'Jn. On the upper Missouri River, Lake Oaha alone has inundated 121,500 ha OO() ,000 acres) of land, including aU riparian areas along a 322 km (200 mile) reach of the r1 ver. Nowhere is the destruction of riparian habitat more dramatic'llly illustrated than in the delta hardwoods of Mississippi where bottomland woodlands were reduced by 60 percent from 1970 to 1976. As development continues nationally, people look for ways tc. enhance stream transportation, maintain or increase water supplies for agricultural, industrial, and urban uses, increase crop production, and protect crops, homes, and industries from flooding. Many combina,tions of draining, diking, diverting, leveeing, damllling, and channeling are used to accomplish these goals. Even with increased legal protection, additional clearing, conversion and alteration of natural riparian habitat continues at a steady pace. Water diversions, dam and reservoir construction and overgrazing in the west, stream channeliz'ltion and alteration in the prairie states, urban and industrial expansion in the east, drainage and forest land conversion in the south, and o~ny other types of man-caused impacts are eating away at the remaining l.inear IIlileage of relatively undisturbed riparian lands. Structure of Riparia}l EcosystE!mS Structurally, stream ecosystems are divided into the "aquatic or water zone" including the channel substrate and the "riparian or streambank zone." The two zones can be and have been studied Individually or as a combined ecosystem. Each stream ecosystem is usually thought to include one aquatic zone and two streambank riparian zones that are bordered by two drier upland terrestrial zones (Figure 1). Riparian_ones tend to be linearly continuous, but some stream reachE!s are sub}ect to interruptions or discontinuities (see definition below). They are vegetatively more highly developed close to the aquatic zone and generally decrease in definition as one moves up slope onto drier land. Riparian zones often include valley floodplains and may. over long time periods, swell and shrink in size according to wet and dry meteorological cycles. They may be very sharply defined with distinct edges and may be easily recognIzed in