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RIVERINE TERRAIN In some landscapes, stream valleys are small and they commonly do not have well-developed flood plains (area R of the conceptual landscape, Figure 2) (see Box I), However, major rivers (area V of the reference landscape, Figure 2) have valleys that usually become increasingly wider downstream, Terraces, natural levees, and aban- doned river meanders are common landscape features in major river valleys, and wetlands and lakes commonly are associated with these features, The interaction of ground water and surface water in river valleys is affected by the interchange of local and regional ground-water flow systems with the rivers and by flooding and evapotranspi- ration. Small streams receive ground-water inflow primarily from local flow systems, which usually have limited extent and are highly variable season- ally, Therefore, it is not unusual for small streams to have gaining or losing reaches that change seasonally, For larger rivers that flow in alluvial valleys, the interaction of ground water and surface water usually is more spatially diverse than it is for smaller streams, Ground water from regional flow systems discharges to the river as well as at various places across the flood plain (Figure 22), If terraces are present in the alluvial valley, local ground-water flow systems may be associated with each terrace, and lakes and wetlands may be formed because of this source of ground water, At some locations, such as at the valley wall and at the river, local and regional ground- water flow systems may discharge in close proximity, Furthermore, in large alluvial valleys, significant down-valley components of flow in the streambed and in the shallow alluvium also may be present (see Box I), Alluvial valley of the Mississippi River. (Photograph by Robert Meade.) 38