Info

27,500

04173500 Cataloging unit

2,150

1,820

The accounting code is sometimes referred to as basin, and the cataloging unit as subbasin or watershed.

The accounting code is sometimes referred to as basin, and the cataloging unit as subbasin or watershed.

River segments, typically extending between an upstream and downstream tributary juncture, are nested within the network. Individual segments may be from one to tens of kilometers in length, and are the scale at which one would observe major floodplain and channel features. Individual reaches are recognizably homogeneous units within a valley segment. In practice, they often are defined as a repeating sequence of channel units (such as a riffle-pool-run sequence) or by a sampling convention, such as a distance equal to 25 stream widths. A reach can be 100 m or less in length in a small stream, and several kilometers in a larger river. Macrohabitats such as a pool or riffle occur within a reach, and microhabitats may include a gravel patch or a leaf accumulation along the stream margin.

The hierarchical view of river systems also emphasizes that processes acting at upper levels of the hierarchy control features expressed lower in the hierarchy, but not vice versa. Climate, source of flow, geology, and landform exert particular control on the river basin and network development, and set the domain of

FIGURE 1.3 The hierarchical organization of a stream system illustrating how smaller units are repeating elements nested within larger units. Upper hierarchical units exert considerable influence over lower hierarchical units, but not vice versa. (Reproduced from Frissell et al. 1986.)

interacting geomorphic processes that shape channels and features at the segment to reach scale. More locally, the stability of banks and supply of pool-forming wood strongly influence details of channel features and habitats.

1.1.2 Longitudinal patterns

Some changes that occur along a river's length have already been noted. An increase in size and volume of water will occur as tributaries join and the network's drainage area increases. Rivers have a characteristic longitudinal profile in which they typically are steeper in the uplands where they originate and have a more gradual slope in the lowlands near their terminus. The longitudinal profile of a river can be divided roughly into three zones: erosion, transfer, and deposition of sediments (Schumm 1977). In addition to their steeper gradients, headwaters often have deep, V-shaped valleys, rapids and waterfalls, and export sediments. The mideleva-tion transfer zone is characterized by broader valleys and gentler slopes. Tributaries merge and some meandering develops. Sediments are received from the headwaters and delivered to lower sections of the river system. In the lower elevation depositional zone, the river meanders across a broad, nearly flat valley, and may become divided into multiple channels as it flows across its own deposited sediments.

This description adds another perspective to our view of rivers, that of sediment erosion, transport, and deposition. Because the river's power to transport sediment is a function of gradient and volume of flow, and more power is required to move large versus small particles, the river also is a sediment sorting machine. Indeed, many of the channel types and features that contribute to the variety of rivers, such as boulder cascades, rapids, riffle-pool sequences, and so on can be seen to exhibit a longitudinal progression determined by sediment supply, stream power, and additional factors considered in Chapter 3 (Figure 1.4).

1.1.3 The stream and its valley

Stream ecologists have long recognized the profound influence that surrounding lands have on the stream ecosystem ("in every respect, the valley rules the stream'', Hynes 1975). Rain and snow that fall within the catchment reach the stream by myriad flow paths. Some, notably surface and shallow subsurface flows, reach the stream rapidly and so high flows quickly follow storms. Others, primarily groundwater flows, are so gradual that streamflow barely responds to rain events. Geology, slope, and vegetation strongly influence these flow paths in natural systems, and human land use adds further complications. Precipitation that flows quickly over surfaces can wash sediments and organic matter into streams, whereas water that has a longer residence time in the ground has more opportunity to dissolve minerals and take on the chemical signature of the underlying geology. The valley slope is the source of much of the sediment inputs in the headwaters, which exports all but the largest particles downstream, and the channel walls become increasingly important as a sediment source in middle and lower river sections as the river's meandering and flooding drives endless cycles of erosion and deposition. Thus, key aspects of the river's hydrology, its channel shape, and its chemistry are the consequences of climate and the geology, topography, and vegetation of the valley.

Land that borders the stream undoubtedly has the greatest influence, affecting multiple stream functions. Called the riparian zone, and including the floodplain in locations where the river frequently overflows its banks, the influence of the stream margin and its vegetation cannot be overstated. Roots stabilize banks and prevent slumping, branches and trunks create habitat diversity wherever they fall into streams, shade from the canopy prevents excessive warming, and the infall of vegetation and invertebrates are major sources of energy to stream food webs. When vegetation changes along a river's length, across

FIGURE 1.4 River channel types occur in succession along the river's profile due to complex interactions governed by slope (s), sediment supply, trapping of sediments by large wood in the channel, and other factors. Although thresholds may be difficult to detect, certain channel features prevail over a substantial distance, referred to as a process domain. This model was developed for small mountain streams in the Western United States. (Reproduced from Montgomery and Buffington 1997.)

FIGURE 1.4 River channel types occur in succession along the river's profile due to complex interactions governed by slope (s), sediment supply, trapping of sediments by large wood in the channel, and other factors. Although thresholds may be difficult to detect, certain channel features prevail over a substantial distance, referred to as a process domain. This model was developed for small mountain streams in the Western United States. (Reproduced from Montgomery and Buffington 1997.)

ecological regions or due to human activities, the stream is affected in multiple ways.

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