It is almost immediately apparent to anybody who studies the Yellow River that its problems can never be fundamentally solved without reducing sediment in the river by controlling erosion in the loess region. Human efforts to restrain nature's plain-building process by channeling river flows with levees or de-silting reservoirs can
only provide temporary and marginal solutions; the key effort must rest with reducing erosion and sediment.
It is generally estimated that the area of high sediment yield in the middle Yellow River basin, with an annual erosion rate exceeding 1,000 t per sq km, extends for 430,000 sq km, over a landscape which is sparsely populated and totally dissected by gullies. The mature gullies reach heights of 200 to 300 m.
Due to the enormous scope of achieving complete erosion control and little immediate economic return to pay for the expenditure, erosion control and sediment control are not treated as an engineering project to be completed with targeted objectives and dates. Rather, they are left as incidental tasks to be accomplished whenever or wherever they can produce agricultural benefits. This policy accounts for the relatively slow rate of progress in the reclamation of the loess region. This article demonstrates that it is feasible to achieve comprehensive sediment control for the entire middle Yellow River valley by treating it like an engineering project with modern construction methods to be completed within, say, 20 years at a cost comparable to a single hydroelectric project.
In 1955, a department under the Yellow River Conservancy Commission, the Upper and Middle Yellow River Administrative Bureau was established, which was given the responsibility and resource to initiate soil conservancy and sediment reduction projects across the loess region. Ever since the early 1950s hillside terracing had been greatly promoted as it created arable land and at the same time reduced erosion damage. The drawback to terracing is its labor-intensive nature and the relatively low productivity of the terraced plots. As most of the loess highland is only thinly populated, it is not likely that terracing can be an effective means in implementing comprehensive erosion and sediment control. Even in densely populated areas it is becoming increasingly difficult to mobilize peasants to contribute their labor, with minimal compensation, to terrace the gully slopes. Added acreage from terracing has declined rapidly in recent years.
Development of grassland is most successful in the relatively flat terrain where ground water is plentiful, as in the regions around the northern bend of the Yellow River in Inner Mongolia, where natural grassland already exists. Large-scale grass or shrub planting and afforestation in the badly dissected loess plateau achieve only limited success due to the aridity of the region; the reported survival rate of new trees is only 20 per cent. Damages done to the grassland by goat-herding and reckless destruction of young trees for fuel also contribute to the effort's demise. Furthermore, over terrain dissected almost totally by gullies with steep slopes and poor water retention, grass, shrubs, and trees have difficulty taking roots. However, erosion is actually the most severe on such terrain, and thus these plantings alone, without subordinate measures, would not be effective in achieving comprehensive erosion control.
A more promising approach is to improve first the productivity of the land with sediment plots by damming gullies with 2 to 5 m-high earth dams, called silt-trapping dams (or warping dams), trapping sediment from eroded topsoil behind them and forming high-yield plots within a few years. These dams can be built at a relatively low cost, one upstream from the other in a staging manner, and the peasants much preferr them to the terraced plots. Driven by economic incentives, local initiatives are propagating the sediment plots with only limited assistance by the government. Their development contributes substantially to reductions in sediment entering the rivers.
In the 1970s, more than 10,000 hectares of sediment plots was created. However, these plots were not intended to withstand severe storms, and consequently a 10-year storm could cause widespread damage to the sediment plots. For instance, in August 1973, 3300 out of 7570 existing plots in the Yanan region were damaged during a single 10-year storm [1]. For maximum survivability, the sediment plots must extensively cover the entire gully system so as to lessen runoff concentration during an intense rainstorm.
To fully develop the benefits of the sediment plots, the concept of a core project had been advocated ever since the 1980s. A core project applies to a major gully with a watershed of 3 to5 sq km, which will be provided with a 30 to50 m-high key dam. Its reservoir, with an average capacity of one million cu m, is adequate to withstand exceptional storm runoffs of the 200-year variety even after 15 years of sediment accumulation (see next section). Hillsides surrounding the reservoir will be terraced, creating irrigated high-yield plots, which in turn will help to reduce slope erosion and silting of the reservoir. Sediment plots created downstream from the dam will be protected by the dam from severe runoffs, and those created upstream from the dam in the branch gullies will trap sediment from reaching the dam reservoir.
This concept of "agricultural oases" is being recognized as a promising development in this region. By promoting the extensive construction of such projects in areas with erosion rate exceeding 5,000 tons per sq km, sediment entering the rivers downstream can be greatly reduced, providing an effective means of sediment reduction as well as flood control for the Yellow River in its lower reaches. This scheme has the prospect of self-propagating once benefits derived from water conservation and productivity of the high-yield plots justify the project's construction cost.
While the construction of core projects based on local initiative provides a viable means in reducing sediment in the rivers, its implementation relies however heavily on the demographics and economic development of the region. With 80% of the heavily dissected loess plateau unpopulated, or thinly populated, it is unlikely that sediment in the Yellow River can be much reduced by such regional efforts in the near future. A major influx of funds from the central government for the region's reclamation to promote economic development and population settlement is needed for achieving the final sediment control in the Yellow River.
Though the investment for such a program is large, the return is many times larger, and is realized in increased water retention for the middle Yellow River basin, resulting in a huge increase in arable land, a better livable environment, a more successful afforestation program, and an expanded economy. In a broader picture, by intercepting flood runoff in the middle reaches of the river high-water threats in the levee-protected reaches of the river are greatly diminished, providing savings on levee improvement and upkeep and on maintenance of flood detention facilities.
At the present time, the main channel downstream often runs dry in March and has very low volume during the summer months, when water is badly needed for irrigation. By regulating water storage in the middle reaches, the flow pattern there will be improved, making avaiable water for irrigation of the summer crops in the neighboring densely-settled provinces. By gradually reducing sediment, the health of the main channel will be steadily improved. The volume of water presently wasted on transporting the suspended silt out to sea may be converted to other uses, reducing the burden on the costly water-diversion program that is being considered.
To be sure, an extensive reclamation program of this scale will modify the environment for the entire river valley and create many ecological and safety issues. The acceptance of such a program is based not only on the economic benefits that it will bring, but also on the sacrifices that the local population are willing to bear. These issues shall be discussed in a separate article. We mention here, however, one major adverse environmental concern requiring attention; that is the excessive amount of soluble salt that is expected to be leached out of the reservoir foundations and be deposited in rivers downstream. Since a large portion of the water from these rivers will be used for irrigation, much of that salt will be transferred to farmland adding to the salinity of the soil. Clearly, many more environmental parameters must be carefully monitored, and their causes and effects examined in detail before the project can be launched. We invite all experts to contribute to this monumental problem of historic proportions.
References
[1] Upper and Middle Yellow River Administrative Bureau, Huanghe Shui Tu bao Chi Zhi (History of Water and Soil Conservation in the Yellow River Valley, Vol. 8 of The Yellow River Compendium), Henan People's Publishing House, Zhengzhou, China, 1993.
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