83
steamboat traffic, primarily during the early decades of the nineteenth century
(Wohl 2014), and continues at present as part of the extensive transportation network for barges (Wohl 2011). Millions of naturally occurring pieces of large wood
were removed from the river and many millions more of cut logs were rafted downstream to collection booms and sawmills (Wohl 2014). The river was dredged,
straightened, channelized, and leveed.
Longitudinal connectivity within the drainage basin is highly reduced as a result
of eight major dams on the Missouri River, 20 locks and dams on the Upper
Mississippi (Fig. 3.14), 21 locks and dams on the tributary Ohio River, eight locks
and dams on the tributary Illinois River, nine locks and dams on the tributary
Tennessee River, and so on through a long list of substantial tributaries. Water and
sediment movements in the river network of the Mississippi River drainage are
highly regulated and bear little resemblance to natural fluxes (Knox 2007). Before
1900, the Mississippi transported an estimated 400 million metric tons of suspended
sediment to the coast, but by 1987–2006 transport averaged 145 million metric tons
per year (Meade and Moody 2010). By 2011, median sediment load just above the
river delta was 73% below the load carried before all of the dams were built
(Heimann et al. 2011). About half of the decline results from sediment trapping
behind dams, with the remainder resulting from the combined effects of channel
stabilization and upland soil erosion controls (Meade and Moody 2010).
The Mississippi drainage also hosts some of the most extensive agricultural
lands in North America and receives associated pesticides and fertilizers in runoff
from these lands, as well as several major urban and industrial areas that contribute
Fig. 3.13 The Mississippi River basin within the conterminous United States. Different subbasins and primary rivers indicated
3.3 Cumulative Effects
steamboat traffic, primarily during the early decades of the nineteenth century
(Wohl 2014), and continues at present as part of the extensive transportation network for barges (Wohl 2011). Millions of naturally occurring pieces of large wood
were removed from the river and many millions more of cut logs were rafted downstream to collection booms and sawmills (Wohl 2014). The river was dredged,
straightened, channelized, and leveed.
Longitudinal connectivity within the drainage basin is highly reduced as a result
of eight major dams on the Missouri River, 20 locks and dams on the Upper
Mississippi (Fig. 3.14), 21 locks and dams on the tributary Ohio River, eight locks
and dams on the tributary Illinois River, nine locks and dams on the tributary
Tennessee River, and so on through a long list of substantial tributaries. Water and
sediment movements in the river network of the Mississippi River drainage are
highly regulated and bear little resemblance to natural fluxes (Knox 2007). Before
1900, the Mississippi transported an estimated 400 million metric tons of suspended
sediment to the coast, but by 1987–2006 transport averaged 145 million metric tons
per year (Meade and Moody 2010). By 2011, median sediment load just above the
river delta was 73% below the load carried before all of the dams were built
(Heimann et al. 2011). About half of the decline results from sediment trapping
behind dams, with the remainder resulting from the combined effects of channel
stabilization and upland soil erosion controls (Meade and Moody 2010).
The Mississippi drainage also hosts some of the most extensive agricultural
lands in North America and receives associated pesticides and fertilizers in runoff
from these lands, as well as several major urban and industrial areas that contribute
Fig. 3.13 The Mississippi River basin within the conterminous United States. Different subbasins and primary rivers indicated
3.3 Cumulative Effects