headward erosion問題............

headward erosion其實係指一條河流的源頭的泥土比較鬆散,容易被水份帶走,加上泥土中的礦物質被溶解(dissolved),久而久之,河道源頭的泥土被侵蝕(eroded),河流露出到表土(ground surface)的部分多了,令河流的長度増加→lenghten the river


Broken rock and soil at the source are carried away by the river, causing the source of the river to move backwards as erosion takes place in the opposite direction to the river's flow. The resulting lowering of the land behind the source may, over time, cause the river to cut backwards.


you can see the video here to see the process:
http://www.classzone.com/books/earth_science/terc/content/visualizations/es1305/es1305page01.cfm?chapter_no=visualization