River Ecology and Restoration - River Dynamics and Integrated River Management - page 656

Fig. 10.33 Riparian forest has been replaced by buildings and the banks are hardened with concrete in Guangyuan,

Sichuan

Tillage and soil compaction interfere with the soil's capacity to partition and regulate the flow of water
in the landscape, increase surface runoff, and decrease the water-holding capacity of soils. Tillage also
often aids in the development of a hard pan, a layer of increased soil density and decreased permeability
that restricts the movement of water into the subsurface. Disturbance of soil associated with agriculture
generates runoff polluted with sediment, a major nonpoint source pollutant in the world. Pesticides and
nutrients (mainly nitrogen, phosphorous, and potassium) applied during the growing season can leach
into ground water or flow in surface water to stream corridors, either dissolved or adsorbed to soil
particles. Improper storage and application of animal waste from concentrated animal production facilities
are potential sources of chemical and bacterial contaminants to stream corridors.
Tree removal decreases the quantity of nutrients in the watershed since approximately one-half of the
nutrients in trees are in the trunks. Nutrient levels can increase if large limbs fall into streams during
harvesting and decompose. Conversely, when tree cover is removed, there is a short-term increase in
nutrient release followed by long-term reduction in nutrient levels. Removal of trees can affect the quality,
quantity, and timing of stream flows. If trees are removed, from a large portion of a watershed, flow quantity
can increase accordingly, and water temperature can increase during summer and decrease in winter.
Many of the potential effects of land use change are cumulative or synergistic. Restoration might not
remove all disturbance factors; however, addressing one or two disturbance activities can dramatically
reduce the impact of those remaining. Simple changes in management, such as the use of conservation
buffer strips in cropland or managed livestock access to riparian areas, can substantially overcome
undesired cumulative effects or synergistic interactions.
Domestic livestock— Stream corridors are particularly attractive to livestock for many reasons. They
are generally highly productive and provide ample forage. Husbandry development in a watershed has
applied a unique stress on the ecosystem. For instance, the riparian vegetation succession from
herbaceous to shrubs has been delayed or even stopped by grazing of livestock along the Ake River on
the Qinghai-Tibet Plateau, as shown in Fig. 10.34(a). On the other hand, the activities of livestock have
become an important element of the river ecology. Excrement of cattle provides the main nutrient for the
grassland. The positive and negative effects of grazing of domestic livestock must be considered in any
restoration strategy. In many cases livestock swimming in a stream can result in extensive physical
disturbance and bacteriological contamination, as shown in Fig. 10.34(b).