92 • Wetlands: Their Use and Regulation 



Figure 7.— Freshwater Wetland Trends (mid-l950's to mid-l970's) 



12,000 -11,720 



10,000 



" 8,000 



a 6,000 



4,000 



2,000 



□ □ 



Actual 

 loss 



Actual 

 gain 



1,828 



899 925 



Agriculture Urban Other use 



SOURCE: USFWS National Wetland Trends Study, 1982 



Lakes Open water 



Conversions of wetlands 



Bare stiore Ottier nonveg. Salt. veg. 



Table 12.— Probable Causes of Freshwater Vegetated Wetland Changes 



Acres Cause of loss 



Freshwater wetland loss to: 



Agriculture 11,720,000 Drainage, flooding, excavation, clearing, land-leveling, filling, ground 



vi^ater pumping, and surface water diversions for conversion to 

 cropland 



Urban use 925,000 Fill for development 



Deep water 621,000 Impoundments 



Otfier use 618,000 Drainage, excavation, filling for forest management, mining, other 



Open water 579,000 Impoundments, drainage/flooding, excavation, climatic changes 



Unconsolidated shore 188,000 Impoundments, grazing, plowing, climatic changes 



Other nonvegetated 25,000 — 



Saltwater vegetated 1,000 Decreased freshwater outflow, destruction of dikes 



Total 14,677,000 



Acres Cause of gain 



Freshwater wetland gains from: 



Other uses 1,828,000 Succession around margins of newly constructed farm ponds 



Agricultural use 899,000 Lack of maintenance on drainage ditches, dikes 



Open water 450,000 Succession around margins of existing ponds 



Deep water 305,000 Succession around margins of larger water bodies 



Unconsolidated shore 65,000 Vegetation establishment 



Urban use 38,000 Drainage and open space management 



Saltwater vegetated wetlands 25,000 Increased freshwater outflow, construction of dikes 



Other nonvegetated 1 2,000 — 



Total 3,622,000 



SOURCE; Data from FWS National Wetland Trends Study, 1983. 



