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Slugs of sediment derived from an episodic landslide may take many years to move down the river corridor, influencing riparian areas to different extents as they pass through specific segments. Large tree boles eroded from riparian areas substantially increase the variation in sediment transport and deposition, and thus also the variety of habitat types available for biota Naiman et al. Seasonal high flows continually disturb new areas. Newly deposited sediments on floodplains undergo biogeochemical changes i.
The overall result is the creation of a complex patchwork of riparian areas, each with a slightly different microenvironment of sediment grain size and nutrient and water availability, and each at a different stage of development since the last disturbance Amoros et al.
Because of the dynamic flow and sediment transport regimes often associated with riparian areas, their soils reflect a high degree of unevenness in particle sizes, soil depth, and the amount of associated compounds such as organic matter. However, in instances where floodplains have been slowly built up via the incremental deposition of fine silt layers over many centuries, soil characteristics across extensive areas may be relatively uniform.
Floodplain soils have been some of the most productive areas in the nation for agricultural production due to their high levels of nutrients and organic matter. Although the energy from water moving down a channel can be used to do work e. Thus, streamside riparian areas are responsible for the dissipation of energy associated with flowing water. Cowan identified several major channel conditions that affect roughness: bed material, degree of surface irregularity, variations in channel cross section, relative effects of obstructions, degree of meandering, and effects of vegetation.
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Importantly, vegetation can directly or indirectly affect all these conditions, with the possible exception of bed material, thus indicating it often has a major influence on channel roughness and on how channels dissipate stream energy during periods of high flow.
Herbaceous riparian vegetation increases local friction on streambanks by creating flexible and three-dimensional barriers to flow. Riparian graminoids grasses, sedges, rushes and shrubs are particularly effective at trapping sediments during high flows and helping to maintain stable streambanks. For forest floodplains, roughness increases directly with the density and size of trees Li and Shen, ; Petryk and Bosmajian, Large wood provided to streams and rivers from riparian forests can also have a significant effect on.
At high flow, streambanks, floodplains, and their associated vegetation provide resistance to flowing water, thus locally altering patterns of scour, sediment transport, and deposition Sedell and Beschta, For example, low velocity zones have been observed to develop when floods pass through riparian forests, creating sites for the retention of sediment and organic matter and refuges for aquatic organisms Swanson et al.
Floodplain vegetation is especially effective at providing protection from scour, which is why well-vegetated floodplains typically are areas of long-term sediment accumulation. During periods of low flow, woody species have a much less significant effect on flow roughness because of the smaller surface area exposed to surface flow, such that flow resistance tends to be controlled more by the morphology of the channel. In contrast, aquatic macrophytes and graminoids can greatly influence the resistance provided during low-flow periods Kauffman and Krueger, Finally, the uptake and transpiration of water by riparian and upslope vegetation during low-flow periods can alter discharge, thereby influencing aquatic habitat Rothacher, ; Troendle, ; Cheng, ; Keppeler and Ziemer, ; Hicks et al.
Unlike the riparian areas of stream and river lotic environments, riparian areas bordering lakes differ significantly in the energy sources that drive physical mixing Wetzel, In the shallow littoral environments of lakeshores, mixing is generally driven by temperature gradients and storm-generated waves. An important contrast with lotic environments is the type and frequency of waterlevel changes at lakeshores. Seiches, for example, can cause substantial changes in water level over periods of days to weeks at the shores of large lakes without the kinds of erosive forces of floods that affect channel floodplains.
Lakeshores also tend to have much larger water-level changes over longer-term interannual cycles, as determined by interannual variation in climate and the regional water balance. Large reservoirs and other river impoundments used for water storage may exhibit nonseasonal fluctuations in water level, with hydrographs varying erratically under the control of hydropower production or irrigation supply.
Consequently, riparian areas around reservoirs are highly variable and often are composed of non-native, invasive species because they have little long-term continuity in water supply and occur in areas of the landscape that have no legacy of native plant colonization e.
Despite major differences in flow velocities and extent of water-level changes, the shallow littoral environment and riparian areas adjacent to lakeshores have much in common with riparian areas bordering streams. As in streams, a.
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In the case of large lakes with inlets from rivers, alluvial deltas may develop by sediment deposition in the river—lacustrine confluence. Often, river deltas in lakes and reservoirs facilitate robust riparian areas in a manner similar to the islands and low terraces that occur in alluvial rivers. Deltaic riparian areas can be large landforms up to many square miles in size. Few studies have been done in such environments e. In summary, riparian areas are characterized by a spatial and temporal mosaic of conditions reflecting variability in sediment type and particle size distribution, timing of water sources and water quality, and time since disturbance by floods.
Seasonal dynamics in flow and sediment transport constitute the foundation of riparian structure and thus influence the resulting colonization by riparian species and the many functions performed by these areas. Moisture availability and anoxia in riparian soil are additional factors that closely follow the distribution of grain sizes determined by fluvial processes. In many channels, the natural variability of flow has been regulated and sediment inputs have been curtailed downstream of dams and water diversions.
As discussed in Chapter 3 , the influence of humans in regulating river flow has had overwhelming effects on ecological processes in rivers and riparian areas, because of the disruption of flow seasonality, sediment dynamics, and moisture availability. Riparian areas receive water from three main sources: 1 groundwater discharge, 2 overland and shallow subsurface flow from adjacent uplands with additional input from direct precipitation, and 3 flow from the adjacent surface water body.diamecasne.cf
The Rivers handbook: hydrological and ecological principles. Vol.2
The major losses of water from riparian areas include groundwater recharge and evapotranspiration. Plate illustrates these major water flow paths for a streamside riparian area. Both the quality in terms of dissolved and particulate constituents and the timing of water from these sources vary considerably.
For example, the discharge of deep groundwater is on the order of centuries, while overbank flows and intense rainstorms can change flows within minutes.
Winter et al. Streams whose downstream flows increase as a result of groundwater discharge are referred to as gaining streams. In contrast, flow in the channel decreases in the downstream direction in losing streams that recharge the groundwater system. Because of variability in water sources and hydrogeologic properties of aquifers, it is typical for streams to simultaneously experience discharge in one reach while experiencing recharge in others. For example, steep mountain streams gain water by groundwater discharge in their upper reaches and then lose water as they flow out of constricted mountain valleys onto alluvial fans.
Lakes and wetlands share some of the same relationships with groundwater as do streams. Lakes and wetlands commonly discharge and recharge simultaneously in different parts of the system and experience flow reversals seasonally Figure As in streams and rivers, movement of water between groundwater and surface water is influenced by the nature of the substrata and the water elevation in the lake compared with water levels and gradients in groundwater of the adjacent aquifer Sebestyen and Schneider, Water moves from areas of high elevation to areas of low elevation, sometimes involving streams or rivers at inlets or outlets to the lake.
The rivers handbook: Hydrological and ecological principles
Because the majority of riparian areas are associated with stream and river channels, this discussion focuses on interactions between groundwater and river channels rather than lakes. From a relatively large-scale perspective miles or greater , the direction of groundwater flow in the vicinity of rivers is typically associated with patterns of floodplain and channel topography. As a result, flow pathways are seldom entirely parallel or entirely perpendicular to the main channel but instead occur diagonally toward the channel in a downstream direction.
The major controls on orientation of groundwater flow paths are hydraulic properties of aquifer materials, regional gradient, and sinuosity of channel Larkin and Sharp, Groundwater that tends to flow parallel to a channel is referred to as underflow Larkin and Sharp, ; in contrast, groundwater flow perpendicular to and toward the channel is referred to as baseflow Hall, see Figure At much smaller spatial scales, i.
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In some settings, baseflow passes directly through riparian sediments, while in others, baseflow may bypass riparian sediments by flowing through coarse material underneath and discharging vertically from directly beneath the stream bed Phillips et al. This short-circuiting of the root zone can have important implications for the extent of certain transformation processes that occur in riparian areas.
As discussed later, the variation in the specific flow paths characteristic of riparian areas may explain why some buffers are not as effective as others. An often-overlooked aspect of groundwater—riparian—channel interactions is that groundwater discharge is not equivalent along all parts of a channel. Instead, certain channel subreaches tend to collect a significant proportion of all ground-. Groundwater discharge points tend to occur at the upstream ends of pools, the upstream side of meanders, anywhere along the channel thalweg deepest area of central channel , and within side channels or alcoves in streams and rivers Harvey and Bencala, An un-.
The importance of groundwater discharge points along channels is twofold. First, areas along channels that collect groundwater discharge tend to favor establishment of rich riparian vegetation, especially in dry climates where water avail-. FIGURE Examples of A underflow-dominated groundwater movement parallel to the channel and B baseflow-dominated groundwater movement perpendicular to the channel.
Groundwater moves in the direction of decreasing water table contours. B Winter et al. Second, groundwater discharge points tend to have cooler water locally in the channel during summer and warmer water in winter compared with other channel areas.