Geoscience Reference
In-Depth Information
addition, observed recruitment of
Salix
and
Populus
seedlings on freshly deposited or exposed surfaces
after the flood suggests that recovery of riparian
forest would follow soon upon the recovery of the
complex channel. Based on the extent of channel
change observed in the 5-year flood, a period of
about four decades would be sufficient to return
the bed complexity of Deer Creek to something
approaching its complexity before the flood control
project.
Fortunately, the Deer Creek floodplain
remains mostly agricultural so there is a better
opportunity here to reconnect the floodplain and
channel than along most streams in California.
Monitoring geomorphic changes during high
flows demonstrates that Deer Creek is capable
of 'restoring itself' by increasing its channel
complexity through erosion and deposition,
and monitoring hyporheic flows demonstrates
that complex, undulating beds on Deer Creek
experience strong hyporheic exchange and,
consequently, provide significant buffering against
the diurnal fluctuations in water temperature.
deposit dynamically. This requires that the river
has sufficient stream power and sediment load
so that it can erode, deposit and build complex
channel forms through natural fluvial processes.
Some channels have such low gradients and low
sediment loads that recovery of a complex pattern
from a straightened, simplified channel might take
centuries (if indeed it were to be achieved at all),
and in these cases, active intervention may be
justified. The
espace de liberte
approach also requires
that sufficient land be made available for the river
channel to be dynamic, eroding and depositing.
This is more likely in rural settings than urban
ones, but even in a highly urbanized region, it is
often possible to find lands adjacent to the river
channel that are suited for such a zone, whether
agricultural, parking lots, or abandoned industrial
parcels.
In rural areas with high-value crops (such as
the premium wine grapes of the Napa Valley) and
channels with erosion concentrated on meander
bends and other hotspots, typically measured in
metres per decade,
anticipatory management
may be
appropriate. This intermediate approach identifies
sites of likely bank erosion and creates vegetated
setbacks there, rather than re-working the banks
or adopting a uniform setback.
TheAireRiver,ontheedgeofurbanGeneva,and
Deer Creek, deep in the rural Sacramento Valley
in California, both possess the stream power and
sediment load needed to rework their channels into
more complex forms, as demonstrated by changes
documented in recent high flows. Along the Aire,
there is sufficient open land (mostly agriculture)
adjacent to the channel to create an
espace de liberte
in some reaches. Along Deer Creek, flood control
levees must be set back to allow the channel
to move, but this approach promises to improve
habitat
Conclusions
Letting the river restore itself through natural
channel dynamics seems an obvious approach,
both for the likely ultimate success of the
restoration and for cost-efficiency in achieving
the result. While such projects have become
common in Europe since the Water Framework
Directive (Council of the European Communities,
2000), the dominant paradigm in North American
river restoration (written into legal mitigation
requirements in some jurisdictions, such as North
Carolina) remains the creation of a stable, single-
thread channel. In large part this probably
reflects popular misconceptions about fluvial
geomorphology and aquatic ecology when stream
restoration became popular in North America (e.g.
notions that stable channels and stable ecosystems
were desirable), as well as cultural preferences for
such channels (Kondolf, 2006).
In the
espace de liberte
approach, a corridor is
set aside within which the river can erode and
whilst
also
reducing
the
risk
of
levee
failure.
As understanding improves on how fluvial
ecosystems function, it is increasingly clear that the
physical processes of erosion, sedimentation, and
channel migration do a very good job of creating
high quality habitat. The most effective approach to
restoring rivers will often be for us to stand aside,
and give the river its space.
