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triggered by clearcutting can scour river channels to bedrock, removing instream
wood and cobble- to gravel-sized sediment that provides spawning habitat for
salmonid fishes (May and Gresswell 2003). Instream wood can be particularly
important in steep portions of a mountainous river network because the wood creates sufficient flow resistance and obstruction to trap coarse sediment and maintain
an alluvial substrate in river segments that have bedrock beds in the absence of
wood (Massong and Montgomery 2000). Even portions of the Coast Range where
clearcutting ceased several decades ago continue to have relatively planar bedrock
channels with minimal fish habitat because of the continuing high transport capacity
for wood and sediment within the active channel. Effective river restoration must
recognize these historical changes and account for limited retentive ability of the
river segments.
Legacy effects refer generally to historical human alterations that continue to
influence river ecosystems, even decades after the original human activity has
ceased. General categories of legacy effects with respect to sediments (James 2013;
Wohl 2015) include activities that reduce sedimentation within river corridors (e.g.,
levees that eliminate floodplain sedimentation), activities that enhance sedimentation (e.g., mill dams), and activities that contaminate river sediments with pollutants
(Stanley and Doyle 2003). Other past alterations such as channel engineering are
also sometimes described as leaving a legacy on river process and form.
A second reason to explicitly account for changes through time in planning river
restoration is that some rivers undergo repeated alternations in process and form at
timescales driven by external processes such as aperiodic precipitation extremes.
An example comes from the semiarid steppe of the interior western United States.
Rivers of the Great Plains alternate at time periods of several decades between
braided, relatively unvegetated planform and meandering channels with a riparian
forest. The change from meandering to braided occurs abruptly during a large
rainfall- runoff flood that removes riparian forests and widens channels. The transition back to meandering occurs over a period of decades as native cottonwood
(Populus spp.) trees that germinated immediately after the flood gradually stabilize
the floodplain (Friedman and Lee 2002) (Fig. 4.7). Failure to recognize these
repeated alterations in process and form within the river corridor can lead to inappropriate restoration targets (Kondolf et al. 2001).
A third reason to think about river restoration in the context of changes through
time involves the future. Many, if not most, river basins will undergo continued
alteration as a result of changing climate and human activities. Accounting for predicted changes in precipitation and proposed or likely future development may help
to prioritize locations or forms of restoration within a drainage basin, as examined
in more detail in the next section in the context of development space.
One of the most effective tools of integrative planning can be to create a conceptual model(s) of a river ecosystem and use that model to guide specific management
actions. When combined with adaptive management that monitors river response to
management, the conceptual model can be continually revised to reflect increasing
understanding of the river ecosystem. Examples of conceptual models and adaptive
4 Toward Sustainable Rivers and Water Resources
triggered by clearcutting can scour river channels to bedrock, removing instream
wood and cobble- to gravel-sized sediment that provides spawning habitat for
salmonid fishes (May and Gresswell 2003). Instream wood can be particularly
important in steep portions of a mountainous river network because the wood creates sufficient flow resistance and obstruction to trap coarse sediment and maintain
an alluvial substrate in river segments that have bedrock beds in the absence of
wood (Massong and Montgomery 2000). Even portions of the Coast Range where
clearcutting ceased several decades ago continue to have relatively planar bedrock
channels with minimal fish habitat because of the continuing high transport capacity
for wood and sediment within the active channel. Effective river restoration must
recognize these historical changes and account for limited retentive ability of the
river segments.
Legacy effects refer generally to historical human alterations that continue to
influence river ecosystems, even decades after the original human activity has
ceased. General categories of legacy effects with respect to sediments (James 2013;
Wohl 2015) include activities that reduce sedimentation within river corridors (e.g.,
levees that eliminate floodplain sedimentation), activities that enhance sedimentation (e.g., mill dams), and activities that contaminate river sediments with pollutants
(Stanley and Doyle 2003). Other past alterations such as channel engineering are
also sometimes described as leaving a legacy on river process and form.
A second reason to explicitly account for changes through time in planning river
restoration is that some rivers undergo repeated alternations in process and form at
timescales driven by external processes such as aperiodic precipitation extremes.
An example comes from the semiarid steppe of the interior western United States.
Rivers of the Great Plains alternate at time periods of several decades between
braided, relatively unvegetated planform and meandering channels with a riparian
forest. The change from meandering to braided occurs abruptly during a large
rainfall- runoff flood that removes riparian forests and widens channels. The transition back to meandering occurs over a period of decades as native cottonwood
(Populus spp.) trees that germinated immediately after the flood gradually stabilize
the floodplain (Friedman and Lee 2002) (Fig. 4.7). Failure to recognize these
repeated alterations in process and form within the river corridor can lead to inappropriate restoration targets (Kondolf et al. 2001).
A third reason to think about river restoration in the context of changes through
time involves the future. Many, if not most, river basins will undergo continued
alteration as a result of changing climate and human activities. Accounting for predicted changes in precipitation and proposed or likely future development may help
to prioritize locations or forms of restoration within a drainage basin, as examined
in more detail in the next section in the context of development space.
One of the most effective tools of integrative planning can be to create a conceptual model(s) of a river ecosystem and use that model to guide specific management
actions. When combined with adaptive management that monitors river response to
management, the conceptual model can be continually revised to reflect increasing
understanding of the river ecosystem. Examples of conceptual models and adaptive
4 Toward Sustainable Rivers and Water Resources