22
beaver meadows (Naiman et al. 1988a; Polvi and Wohl 2012). Beaver dams create
obstructions to flow that facilitate hyporheic exchange and overbank flooding
(Westbrook et al. 2006, 2013). Overbank flooding enhances infiltration and maintains a high riparian water table (Gurnell 1998). Water, sediment, and organic matter
are stored upstream from dams, in overbank areas, and in the hyporheic zone. This
storage attenuates downstream fluxes of water, solutes, sediment, and particulate
organic matter (Naiman et al. 1986; Pollock et al. 2007, 2014; Johnston 2014).
Water moving across the floodplain and through beaver-dug canals also forms
anabranching channels (John and Klein 2004; Polvi and Wohl 2013). By greatly
increasing the habitat diversity of the river corridor (Burchsted et al. 2010), beaver
increase biodiversity, with beneficial effects on water quality and ecosystem services. Although beaver can only dam small to moderate-sized channels, the animals
can construct extensive dam and pond complexes in the floodplains of very large
rivers. In floodplains, beaver dam secondary channels, tributary channels, and
ground water springs or seeps coming from adjacent hillslopes. In a variety of contexts, beaver influence physical processes and forms in river corridors.
In summary, explicitly recognizing the existence and the details of interconnections within the river corridor is necessary to effectively manage river ecosystems.
River management designed to reduce nitrate levels, for example, requires understanding the role of microbial communities within riparian and hyporheic zones and
the factors that facilitate exchanges between flow in the active channel and the riparian and hyporheic zones.
2.3.2 Energy Transfers Within River Ecosystems
River ecologists conceptualize coupling between the physical environment and
biotic communities in terms of energy flows and disturbance regimes. Energy flows
can be illustrated as food chains, food webs, or trophic cascades (Fig. 2.5). Each
phrase describes the processes by which solar energy is initially captured through
photosynthesis and then transferred among organisms through processes such as
herbivory, parasitism, and predation. Within river channels, energy flows among
organisms start with either autochthonous or allochthonous production.
Autochthonous production occurs within the channel via photosynthesis by bryophytes (mosses and lichens), attached algae and floating or rooted angiosperms
(Ward 1992). Allochthonous production is based on organic detritus from outside
the channel in the form of dissolved and particulate organic matter. Much of this
organic matter entering the channel comes from leaf litter and wood that is broken
down by microbes, fungi, and aquatic insects. Within floodplains, energy flows also
start with allochthonous production by riparian terrestrial or wetland vegetation,
although autochthonous inputs of dissolved and particulate organic matter during
overbank floods can be very important.
The primary consumers of organic matter within a channel are biofilm assemblages, macroinvertebrates, and fish. Biofilms composed of attached algae, bacteria,
2 Rivers as Ecosystems
beaver meadows (Naiman et al. 1988a; Polvi and Wohl 2012). Beaver dams create
obstructions to flow that facilitate hyporheic exchange and overbank flooding
(Westbrook et al. 2006, 2013). Overbank flooding enhances infiltration and maintains a high riparian water table (Gurnell 1998). Water, sediment, and organic matter
are stored upstream from dams, in overbank areas, and in the hyporheic zone. This
storage attenuates downstream fluxes of water, solutes, sediment, and particulate
organic matter (Naiman et al. 1986; Pollock et al. 2007, 2014; Johnston 2014).
Water moving across the floodplain and through beaver-dug canals also forms
anabranching channels (John and Klein 2004; Polvi and Wohl 2013). By greatly
increasing the habitat diversity of the river corridor (Burchsted et al. 2010), beaver
increase biodiversity, with beneficial effects on water quality and ecosystem services. Although beaver can only dam small to moderate-sized channels, the animals
can construct extensive dam and pond complexes in the floodplains of very large
rivers. In floodplains, beaver dam secondary channels, tributary channels, and
ground water springs or seeps coming from adjacent hillslopes. In a variety of contexts, beaver influence physical processes and forms in river corridors.
In summary, explicitly recognizing the existence and the details of interconnections within the river corridor is necessary to effectively manage river ecosystems.
River management designed to reduce nitrate levels, for example, requires understanding the role of microbial communities within riparian and hyporheic zones and
the factors that facilitate exchanges between flow in the active channel and the riparian and hyporheic zones.
2.3.2 Energy Transfers Within River Ecosystems
River ecologists conceptualize coupling between the physical environment and
biotic communities in terms of energy flows and disturbance regimes. Energy flows
can be illustrated as food chains, food webs, or trophic cascades (Fig. 2.5). Each
phrase describes the processes by which solar energy is initially captured through
photosynthesis and then transferred among organisms through processes such as
herbivory, parasitism, and predation. Within river channels, energy flows among
organisms start with either autochthonous or allochthonous production.
Autochthonous production occurs within the channel via photosynthesis by bryophytes (mosses and lichens), attached algae and floating or rooted angiosperms
(Ward 1992). Allochthonous production is based on organic detritus from outside
the channel in the form of dissolved and particulate organic matter. Much of this
organic matter entering the channel comes from leaf litter and wood that is broken
down by microbes, fungi, and aquatic insects. Within floodplains, energy flows also
start with allochthonous production by riparian terrestrial or wetland vegetation,
although autochthonous inputs of dissolved and particulate organic matter during
overbank floods can be very important.
The primary consumers of organic matter within a channel are biofilm assemblages, macroinvertebrates, and fish. Biofilms composed of attached algae, bacteria,
2 Rivers as Ecosystems