64
sediment yield depend on the characteristics of the river corridor. Summarizing multiple studies conducted in mountainous areas of New Zealand, for example, Liébault
et al. (2005) describe how the conversion of forest to pasture between AD 1840 and
1940 resulted in an order of magnitude increase in sediment yield. The sediment was
rapidly transported through the steep, narrow mountainous valleys, however, in contrast to river networks in other regions of the world that have experienced channel
aggradation and widening following deforestation (Liébault et al. 2005).
Changes in land cover alter nutrient yields to the river corridor by altering the
primary productivity of upland vegetation; the nutrient content of soils; and the
transport of nutrients moving downslope with water and sediment. Cropping, in
particular, typically increases nutrient yields because of excess fertilizer transported
from crop fields in dissolved form during runoff (Vitousek et al. 2009), but any
change in land cover can alter nutrient yields.
In regions that are forested under natural conditions, changes in land cover can
also significantly alter the recruitment of large wood to river corridors by increasing
or decreasing forest cover and altering slope stability. Deforestation in regions of
steep terrain, for example, is likely to increase the frequency and size of landslides
and debris flows (Imaizumi et al. 2008). These forms of slope instability can recruit
large wood to river corridors (May and Gresswell 2003), so one indirect effect of
deforestation can be a short-term increase in wood inputs from unstable hillslopes
that have not been completely deforested.
Fig. 3.2 Flow chart illustrating the most common changes in how precipitation enters rivers after
changes in land cover. Upward arrow indicates increase; downward arrow indicates decrease; triangle indicates change (evapotranspiration can either increase or decrease following changes in
land cover, depending on what replaces the native plant community)
3 Human Alterations of Rivers
sediment yield depend on the characteristics of the river corridor. Summarizing multiple studies conducted in mountainous areas of New Zealand, for example, Liébault
et al. (2005) describe how the conversion of forest to pasture between AD 1840 and
1940 resulted in an order of magnitude increase in sediment yield. The sediment was
rapidly transported through the steep, narrow mountainous valleys, however, in contrast to river networks in other regions of the world that have experienced channel
aggradation and widening following deforestation (Liébault et al. 2005).
Changes in land cover alter nutrient yields to the river corridor by altering the
primary productivity of upland vegetation; the nutrient content of soils; and the
transport of nutrients moving downslope with water and sediment. Cropping, in
particular, typically increases nutrient yields because of excess fertilizer transported
from crop fields in dissolved form during runoff (Vitousek et al. 2009), but any
change in land cover can alter nutrient yields.
In regions that are forested under natural conditions, changes in land cover can
also significantly alter the recruitment of large wood to river corridors by increasing
or decreasing forest cover and altering slope stability. Deforestation in regions of
steep terrain, for example, is likely to increase the frequency and size of landslides
and debris flows (Imaizumi et al. 2008). These forms of slope instability can recruit
large wood to river corridors (May and Gresswell 2003), so one indirect effect of
deforestation can be a short-term increase in wood inputs from unstable hillslopes
that have not been completely deforested.
Fig. 3.2 Flow chart illustrating the most common changes in how precipitation enters rivers after
changes in land cover. Upward arrow indicates increase; downward arrow indicates decrease; triangle indicates change (evapotranspiration can either increase or decrease following changes in
land cover, depending on what replaces the native plant community)
3 Human Alterations of Rivers