. Une évaluation de la forêt et des parcours situation aux États-Unis. Les forêts et la foresterie United States ; gestion des forêts des États-Unis ; les parcours des États-Unis. Figure 4.8 État des zones humides dans les États-Unis. Les zones humides plus ttireat face à l'Alaska du drainage y^^tiabitat d'hivernage de la sauvagine Q besoin de protection détruits par fiigtiway ^ et construction d'installations d'eau de reproduction de la sauvagine ont besoin de protection (fiabitat une encroactiment urbain ou industriel membres a entraîné lorsque le prix du blé s'est accéléré après les grandes ventes aux nations du bloc communiste au début des années 70. Riparian

• Contributeur: Library Book Collection / Alamy Banque D'Images
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. An assessment of the forest and range land situation in the United States. Forests and forestry United States; Forest management United States; Rangelands United States. Figure 4.8 Status of Wetlands in the United States. Alaska Wetlands facing greatest ttireat of drainage y^^Q Waterfowl wintering tiabitat needing protection ^ Destroyed by fiigtiway construction and water facilities Waterfowl breeding fiabitat needing protection A Urban or industrial encroactiment States resulted when the price of wheat accelerated after large sales to the Communist bloc nations in the early 1970's. Riparian habitats on rangelands are very suscepti- ble to damage from overgrazing by livestock. Re- moval of tree cover in the understory is of special concern, particularly in the Southwest where this rel- atively limited habitat is vital to the native fauna. Its removal reduces wildlife population levels and uUi- mately the variety of wildlife that can be supported. When riparian lands are grazed too heavily, stream- banks are damaged, movement of sediment into the stream channel is accelerated, stream channels tend to become wider and shallower, and the water becomes warmer. These physical changes may adver- sely affect aquatic organisms. To some degree, down- stream habitats are also affected. Modifications of Aquatic Ecosystems The relative importance of current water-related activities and conditions that have major implications for wildlife and fish is shown by section in table 4.18. The problems of major concern include reductions in streamflows, physical changes to free-flowing streams and rivers, pollution, and sedimentation.3"* The consumptive use of water for urban, agricul- tural, and industrial purposes and the consequent reductions in streamflows is great enough in some areas to threaten the existence of aquatic organisms. This is particularly true in the Southwest and in the southern portions of the Rocky Mountains and Great Plains. In especially dry years, problem

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