21
quality; and ecosystem services. Three examples illustrate this coupling: microbial
communities in the hyporheic zone; woody riparian vegetation; and beaver.
2.3.1 Interconnections Within the River Corridor
Hyporheic exchange exposes nutrients in river water to alternating anoxic and oxic
zones in the streambed. These zones are composed of geochemically reactive sediment and microbial communities (Lautz and Siegel 2007). Among nutrients, nitrogen is of particular concern because of human-induced increases in nitrogen entering
river corridors and the negative effects of excess nitrogen on water quality and
freshwater and marine biotic communities. Only about a fifth of the nitrogen entering rivers is carried to the oceans because of removal and transformation of nitrogen
in river corridors (Van Breemen et al. 2002). Much of this removal and transformation depends on biogeochemical hot spots with accelerated chemical reactions, such
as are present beneath riparian vegetation (Lowrance et al. 1984) and in the hyporheic zone (Harvey and Fuller 1998). Microbes in the riparian and hyporheic zones
are critical to nitrogen removal (Nihlgard et al. 1994), but features such as bedforms
and instream wood that promote hyporheic exchange are equally critical to ensuring
that downwelling occurs and provides nitrogen to the subsurface microbial communities. In this example, physical features of the river corridor control hyporheic
exchanges, but microbial communities control the effects of these exchanges on
water quality.
Woody riparian vegetation also illustrate coupling within the river corridor.
Woody riparian vegetation strongly influences hydraulics and substrate resistance in
many river corridors. The aboveground portion of vegetation growing on the river
banks and across the floodplain increases flow resistance. These effects are illustrated by a case study of the sand-bed Rio Puerco channel in New Mexico, USA,
where dense woody vegetation along the channel banks reduces perimeter-averaged
boundary shear stress by almost 40% and boundary shear stress in the channel center by 20% (Griffin et al. 2005). Vegetation also increases the mass of banks, which
can facilitate bank failure. In general, however, riparian vegetation increases the
erosional resistance of river banks (Merritt 2013) by creating aboveground frictional resistance to flow that reduces velocity and hydraulic force exerted against the
bank, and by increasing the resistance of bank sediment to shearing via the presence
of plant roots (Pollen and Simon 2005). High flows that create local bank erosion
can remove existing riparian vegetation, but also provide germination sites for new
riparian plants. In this example, riparian vegetation both responds to physical processes and alters channel boundaries in ways that influence physical processes.
Beaver (Castor fiber in Eurasia, C. canadensis in North America) provide a third
example of coupling within river corridors. Beaver are the premier ecosystem engineers of river corridors in the northern hemisphere. Although now much less common, hundreds of millions of these animals once built dams across rivers and dug
narrow canals throughout floodplains, creating river corridor wetlands known as
2.3 Rivers as Ecosystems
quality; and ecosystem services. Three examples illustrate this coupling: microbial
communities in the hyporheic zone; woody riparian vegetation; and beaver.
2.3.1 Interconnections Within the River Corridor
Hyporheic exchange exposes nutrients in river water to alternating anoxic and oxic
zones in the streambed. These zones are composed of geochemically reactive sediment and microbial communities (Lautz and Siegel 2007). Among nutrients, nitrogen is of particular concern because of human-induced increases in nitrogen entering
river corridors and the negative effects of excess nitrogen on water quality and
freshwater and marine biotic communities. Only about a fifth of the nitrogen entering rivers is carried to the oceans because of removal and transformation of nitrogen
in river corridors (Van Breemen et al. 2002). Much of this removal and transformation depends on biogeochemical hot spots with accelerated chemical reactions, such
as are present beneath riparian vegetation (Lowrance et al. 1984) and in the hyporheic zone (Harvey and Fuller 1998). Microbes in the riparian and hyporheic zones
are critical to nitrogen removal (Nihlgard et al. 1994), but features such as bedforms
and instream wood that promote hyporheic exchange are equally critical to ensuring
that downwelling occurs and provides nitrogen to the subsurface microbial communities. In this example, physical features of the river corridor control hyporheic
exchanges, but microbial communities control the effects of these exchanges on
water quality.
Woody riparian vegetation also illustrate coupling within the river corridor.
Woody riparian vegetation strongly influences hydraulics and substrate resistance in
many river corridors. The aboveground portion of vegetation growing on the river
banks and across the floodplain increases flow resistance. These effects are illustrated by a case study of the sand-bed Rio Puerco channel in New Mexico, USA,
where dense woody vegetation along the channel banks reduces perimeter-averaged
boundary shear stress by almost 40% and boundary shear stress in the channel center by 20% (Griffin et al. 2005). Vegetation also increases the mass of banks, which
can facilitate bank failure. In general, however, riparian vegetation increases the
erosional resistance of river banks (Merritt 2013) by creating aboveground frictional resistance to flow that reduces velocity and hydraulic force exerted against the
bank, and by increasing the resistance of bank sediment to shearing via the presence
of plant roots (Pollen and Simon 2005). High flows that create local bank erosion
can remove existing riparian vegetation, but also provide germination sites for new
riparian plants. In this example, riparian vegetation both responds to physical processes and alters channel boundaries in ways that influence physical processes.
Beaver (Castor fiber in Eurasia, C. canadensis in North America) provide a third
example of coupling within river corridors. Beaver are the premier ecosystem engineers of river corridors in the northern hemisphere. Although now much less common, hundreds of millions of these animals once built dams across rivers and dug
narrow canals throughout floodplains, creating river corridor wetlands known as
2.3 Rivers as Ecosystems