116
human alterations, even after the human activities have ceased. Sediment slugs in
southeastern Australia, mill dams in the eastern United States, and lack of spawning
gravels in the Oregon Coast Range, USA provide examples.
Clearing of native vegetation by settlers of European descent occurred throughout southeastern Australia between circa 1790 and 1850 (Brierley et al. 2005). As in
other regions of the world, removal of upland vegetation caused increased sediment
yields from uplands to river corridors. Removal of riparian vegetation and instream
large wood reduced physical complexity and flow resistance within river corridors.
In response, channels became wider, shallower, and straighter, with floodplain
aggradation and development of inset benches where sediment accumulated along
the margins of the active channel (Brooks and Brierley 1997; Fryirs and Brierley
2001; Brierley et al. 2005) (Fig. 4.6), as well as plugs of sand at tributary junctions
that create shallow lakes (Robson and Mitchell 2010). River management and restoration in these river networks must account for continuing responses by the river
corridor to historical land uses. In this example, sediment dynamics within river
corridors continue to reflect land uses that occurred more than a century ago.
Mill dams provided power sources for numerous activities prior to the development of steam and electrical power. Working in the Mid-Atlantic Piedmont region
of the United States, Walter and Merritts (2008) document the existence of thousands of mill dams built in close proximity to one another along many rivers, with
the backwater from one dam touching the base of the dam upstream. Each mill
pond collected sediment coming from uplands that were simultaneously being
cleared for agriculture. As other power sources replaced water mills and the mill
ponds completely filled with sediment, the dams were gradually breached and
largely forgotten. Rivers cut down into the thick sequences of ‘legacy sediment’
that had accumulated along the river corridors and people forgot that the dams had
ever been present. Gravel-bed rivers flowing between tall banks of finer sediment
topped with riparian trees came to be viewed as the natural condition for the region
until Walter and Merritts (2008) showed that many of the channels were historically
swampy or marshy swales prior to construction of mill dams. Of particular importance, this work and subsequent studies (Schenk and Hupp 2009; Pizzuto and
O’Neal 2009; Merritts et al. 2013) indicate that much of the fine sediment and nutrient loads causing problems in Chesapeake Bay and other estuaries originates primarily from erosion of these legacy sediments along river corridors rather than
from erosion in upland crop fields. Consequently, river restoration designed to limit
fine sediment and nutrient yields to downstream portions of the watershed has to
focus on the presence and stability of river banks in addition to implementing best
management practices in upland areas. In this example, channel form and sediment
yields to downstream regions continue to reflect land use practices that have long
since ceased.
Commercial timber harvest in the Oregon Coast Range of the western United
States began during the early twentieth century and continued at high intensity until
the 1990s (Gale et al. 2012). Clearcutting, particularly in this wet and tectonically
active environment, substantially increases hillslope instability and water and sediment yields to river corridors. In the upper portions of river networks, debris flows
4 Toward Sustainable Rivers and Water Resources
human alterations, even after the human activities have ceased. Sediment slugs in
southeastern Australia, mill dams in the eastern United States, and lack of spawning
gravels in the Oregon Coast Range, USA provide examples.
Clearing of native vegetation by settlers of European descent occurred throughout southeastern Australia between circa 1790 and 1850 (Brierley et al. 2005). As in
other regions of the world, removal of upland vegetation caused increased sediment
yields from uplands to river corridors. Removal of riparian vegetation and instream
large wood reduced physical complexity and flow resistance within river corridors.
In response, channels became wider, shallower, and straighter, with floodplain
aggradation and development of inset benches where sediment accumulated along
the margins of the active channel (Brooks and Brierley 1997; Fryirs and Brierley
2001; Brierley et al. 2005) (Fig. 4.6), as well as plugs of sand at tributary junctions
that create shallow lakes (Robson and Mitchell 2010). River management and restoration in these river networks must account for continuing responses by the river
corridor to historical land uses. In this example, sediment dynamics within river
corridors continue to reflect land uses that occurred more than a century ago.
Mill dams provided power sources for numerous activities prior to the development of steam and electrical power. Working in the Mid-Atlantic Piedmont region
of the United States, Walter and Merritts (2008) document the existence of thousands of mill dams built in close proximity to one another along many rivers, with
the backwater from one dam touching the base of the dam upstream. Each mill
pond collected sediment coming from uplands that were simultaneously being
cleared for agriculture. As other power sources replaced water mills and the mill
ponds completely filled with sediment, the dams were gradually breached and
largely forgotten. Rivers cut down into the thick sequences of ‘legacy sediment’
that had accumulated along the river corridors and people forgot that the dams had
ever been present. Gravel-bed rivers flowing between tall banks of finer sediment
topped with riparian trees came to be viewed as the natural condition for the region
until Walter and Merritts (2008) showed that many of the channels were historically
swampy or marshy swales prior to construction of mill dams. Of particular importance, this work and subsequent studies (Schenk and Hupp 2009; Pizzuto and
O’Neal 2009; Merritts et al. 2013) indicate that much of the fine sediment and nutrient loads causing problems in Chesapeake Bay and other estuaries originates primarily from erosion of these legacy sediments along river corridors rather than
from erosion in upland crop fields. Consequently, river restoration designed to limit
fine sediment and nutrient yields to downstream portions of the watershed has to
focus on the presence and stability of river banks in addition to implementing best
management practices in upland areas. In this example, channel form and sediment
yields to downstream regions continue to reflect land use practices that have long
since ceased.
Commercial timber harvest in the Oregon Coast Range of the western United
States began during the early twentieth century and continued at high intensity until
the 1990s (Gale et al. 2012). Clearcutting, particularly in this wet and tectonically
active environment, substantially increases hillslope instability and water and sediment yields to river corridors. In the upper portions of river networks, debris flows
4 Toward Sustainable Rivers and Water Resources