Stream hydrology, the daily and seasonal pattern of a stream's discharge, is one of the most managed components, although some features, such as extreme floods and drought, are difficult or impossible to control. Hydrology is also one of the most important components to ecological self-organization and stability. Hydrology shapes the physical habitat within a channel by creating pools, riffles, and meanders, regulating substrata size and sediment load, and establishing a relatively stable downward slope across the landscape. Natural stream discharge, which shapes biotic communities through seasonal floods and droughts, has spatial and temporal variations in channel velocity, material transport, and headwater-downstream linkages of energy flow. Alterations to the physical characteristics of the channel, or to the amount or timing of the discharge, can affect the natural variability, and in turn the biotic communities that have adapted to it.
Changes in hydrology can be caused by channelization, dams, and/or watershed alteration from urbanization, deforestation, or agricultural development. The removal ofnative vegetation within the watershed or in key riparian areas and the construction of impervious surfaces strongly influence seasonal and daily discharge and increase the intensity and frequency of flooding. During storm events, reduction in precipitation infiltration, vegetation interception, and evapotranspiration increases overland flow (runoff) causing water to enter the stream quicker and in a larger pulse. On the other side of the spectrum, water withdrawal and dam construction can also cause major changes in natural discharge. Surface water and ground-water extraction for irrigation, domestic, and industrial use can lead to significantly reduced channel flows or loss of channel discharge altogether. Dams cause extensive ecosystem changes due to the considerable alteration of natural daily and seasonal discharge patterns, disruption of biological river connectivity (e.g., stopping upstream movement of spawning fish), as well as reduction of natural sediment transport, which would otherwise shape instream habitat.
All stream ecosystem processes are related to discharge. Water movement transports dissolved nutrients and particulate organic matter, and removes waste from the system. It shapes the stream channel creating habitat diversity, brings in new colonists, and allows for the migration of species across the landscape.
Stream geomorphology is the development and subsequent changes of a channel's physical dimensions over time. Stream channels are naturally altered by the dissipation of energy from moving water. Regional underlying geologic features are affected differently by flowing water, so a large portion of a channel's inherent stability is dependent upon its local geologic history. For example, historically, low-gradient, meandering streams such as the lower Mississippi River, USA, that flows through fine alluvial deposits, have a high natural sinuosity with relatively low channel stability over geologic time. In contrast, approximately 2000 km away, is the steeper sloped Colorado River, which drops over 3000 m in elevation from the headwaters to the delta. This river flows through uplifted erodible sedimentary deposits of sandstone, siltstone, and shale and has created a relatively stable, highly incised channel.
Channels can be grouped into four general geomor-phologically based classes: straight, meandering, braided, and anastomosing. Although helpful for differentiating general stream geometry, there is little correlation between stream class and sensitivity to anthropogenic disturbances. At a local scale, stream channels can be defined by characteristics such as length, width, depth, cross-sectional area, slope, and particle size, all of which can be directly affected by channel, floodplain, and watershed development.
A stream meanders to equalize the dissipation of energy of the flowing water, and produce an even slope as it flows through a basin. Channelization and the addition of levees on many streams have occurred in the name of flood control. Channel straightening reduces the longitudinal distance, which increases the slope between two points (i.e., increases erosion potential), reduces the pool to riffle distance ratio, reduces the volume of water a section of stream can hold, and reduces stream habitat such as woody debris. Straightening of a channel increases the velocity through the channel, which can result in increased suspended sediment and bedload, and downstream flooding (Figure 1).
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X- Points of channel erosion (energy dissipation)
Figure 1 Example of some physical changes that occur with channelization. After channelization, the channel distance between two points is decreased, but the elevation remains the same. With the increased slope and reduction of channel bends, the potential energy of the flowing water switches from a lateral transfer of energy and sideways movement of the channel, to a downward erosional pattern and deepening of the river channel.
The biological consequences of channel modification can be significant, affecting bacteria, algae, macroinverte-brates, mussels, benthic fishes, and littoral macrophytes. Constraining the channel also reduces the interaction between the stream and its floodplain, which can be a major source of nutrient exchange. Autotrophic microbial assemblages can be altered by reductions in light availability caused by increased turbidity, and loss of shade due to riparian vegetation reduction. Reductions in allocthonous carbon from riparian vegetation, changes in substrata composition (e.g., hard and stable to soft and silty), and changes in current velocity can alter the entire benthic community.
Water temperature and light affect both biological and chemical processes in aquatic systems. Although natural seasonal variation in these parameters can be great, resident aquatic organisms have evolved to deal with these regular fluctuations, by regulating metabolism, annual reproductive cycles, and changes in pigment concentration. Thermal deviations from this natural variation can occur locally due to an input of industrial or municipal wastewater, power plant effluent, increased solar input from the removal of riparian canopy shade, or reduced from input of groundwater. On both local and regional scales, hypolemnetic release dams can lower stream water temperature drastically by releasing colder water from the bottom of a reservoir, or increase it by releasing warmer water from the epilimnion.
Increased water temperature can increase the rate of metabolic activity within a system, leading to faster microbial nutrient cycling, and altered reproductive success and juvenile development of aquatic plants, macroinvertebrates, mussels, and fish. Light availability in streams can be lowered by excessive sediment loads caused by dredging, watershed erosion from agricultural practices, deforestation, or urban development. Depending on stream size, the loss of riparian cover can also increase light availability to the channel. Light availability and primary productivity are directly linked within aquatic system.
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