Valley head erosion

Japanese: 谷頭浸食 - こくとうしんしょく

This refers to the erosion that takes place at the head of a valley, above the water source, in other words, at the uppermost part of the valley. It is also called head erosion or retreat erosion. Normally, no flowing water is seen at the head of the valley, but when it rains, a sheetflood occurs, which flows like a cloth without forming a stream, washing away weathered and collapsed materials from the slopes of the valley walls. In addition, groundwater welling up from a spring just downstream of the head of the valley undermines the strata above the spring. This process causes the slope of the head of the valley to retreat upstream, causing the valley to extend upstream.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

谷の最上流部で水源より上方の部分，すなわち谷頭で行われる浸食作用をいう。頭部浸食，後退浸食ともいう。谷頭ではふだん流水を見ないが，降雨時には水流を形成せずに布状に面として流れる布状洪水sheetfloodが発生して谷壁斜面の風化物質や崩壊物質を洗い流す。また，谷頭のすぐ下流側にある泉からの地下水の湧出により，泉の上部にある地層が掘り崩される。このような作用により谷頭部の斜面は上流側に後退するので，それに伴って谷が上流側へ延びていく。

出典　株式会社平凡社世界大百科事典第２版について　情報

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