Forest Waterfalls

Forest Waterfalls 

basalts is most intense in two nearly (but not quite) parallel directions, and the river has cut a zigzag path working upstream, first along one direction of weakness, then the other. Presently it appears to be just at the point where it will take another sharp bend. Other examples of waterfalls produced by uplift of plateaus include Angel Falls in Venezuela—the highest in the world at 977 meters (3,212 feet)—and the collection of smaller waterfalls along the "fall line" at the eastern edge of the Appalachian Mountains. Situated on the border of Brazil, Argentina, and Paraguay, Iguazu Falls consists of 275 cascades spread across nearly 3.3 kilometers (2 miles). With an average (nonflood) discharge of 1,700 cubic meters per second (60,000 cubic feet per second), these falls are among the most powerful in the world. Situated on the border of Brazil, Argentina, and Paraguay, Iguazu Falls consists of 275 cascades spread across nearly 3.3 kilometers (2 miles). 

        With an average (nonflood) discharge of 1,700 cubic meters per second (60,000 cubic feet per second), these falls are among the most powerful in the world. River Diversion. Niagara Falls is the archetype for the second kind of waterfall; namely, one produced by the diversion of a preexisting river. The ancestral Niagara River followed a course more or less parallel to the current river for part of its course, and carved a gorge similar in scale to the modern gorge. During the last ice age, the old gorge was filled with glacial debris, diverting the river into a new path across the dolomite upland. 

     The new gorge began at the escarpment near Queenston, Ontario, which is where the diverted (modern) Niagara River fell over the edge of this upland. * Away from the Niagara escarpment, the hard dolomite shielded the underlying soft shales from erosion. Flowpaths beyond the escarpment traversed areas not protected by this dolomite cap, and hence gradually eroded the underlying glacial deposits. Over thousands of years, the differential erosion created a vertical waterfall. It is estimated that 12,000 years ago, the falls were 11 kilometers (7 miles) downstream from their present position. The continuous removal of the shales at the base of the falls has steadily undermined the dolomite cap, causing its collapse and hence the ongoing retreat of the falls upstream. Today the erosion continues, but human modifications of the river's flow have reduced the erosion rate. 

       Differential Valley Erosion. Probably the best example of the last type of waterfall is Yosemite Falls in California. Yosemite Falls (and many of the other famous waterfalls in Yosemite National Park) is the result of the powerful erosion of Yosemite Valley by a glacier flowing down from the High Sierra. The glacier in the main valley was larger, and especially thicker, than the tributary glaciers that flowed into it. The thick ice stream carved a deep, flat-floored valley, and was much more effective in doing this than the smaller, thin tributary ice streams. When the ice melted away, the result of the differential erosion between the main and tributary glaciers left the floors of the smaller glacial valleys perched high above the main valley floor. Rivers such as the Yosemite now leap over immense drops to meet the Merced River, which flows at the bottom of the main valley. 

       Ongoing Evolution In all of these examples, the geologic activity producing the waterfalls is quite recent—perhaps 10,000 to 15,000 years for Niagara Falls and Yosemite, and probably somewhat longer in the case of Victoria Falls. However, the uplift of east Africa still may be occurring, likely at a rate of 2 to 3 centimeters (about 1 inch) each year.