Honshu's Rivers

Due to the steep, mountainous topography of much of Honshu, many of the rivers are regarded as some of the fastest flowing in the world. These mountains are highly impermeable, which decreases absorption, which in turn allows for greater drainage and runoff.

Mount Kobushi showing the steepness of the mountains in Central Honshu 

The mountains tend to drain radially, where water flows from a a central high point to a lower point. Although a vast majority of Honshu receives little rain (save for the rainy season), they experience heavy snowfalls, which create deep snowbanks in the mountainous regions (1) allowing for huge runoff as it melts. These flows eventually lead to a dendritic pattern of rivers, with many of Honshu's larger ones feeding smaller ones.

Radial drainage patters from Mount Shirouma & Mount Korenge

Radial drainage from Mount Fuiji.

Since these flows are so heavy, the carve out valleys such as the Kurobe Gorge, created by the Kurobe River (2), collecting sediments and minerals as they flow downriver. Likewise, these rivers flow through vast amounts of forests, again adding to the sediments collected. As these steep, mountainous rivers met flatter areas, they deposited these minerals and sediment, over time creating alluvial fans. Because they contain a high mineral count, alluvial fans are very fertile regions and good for growing crops, such as rice.
For example, in the Kanto Region of Honshu, they are fed by the Tone River, which in turn is fed by several tributaries- the Agatsuma, Watarase, Kinu, Omoi, and Kokai Rivers. Together, with their high mineral count, feed the alluvial fan of the Tama Hills in Kanto's plains (3), thus providing vital irrigation to the rice paddies in Kanto.

The Tone River and its tributaries.

Source:

1) http://www.jnto.go.jp/eng/arrange/essential/climate.html

2)http://www.britannica.com/EBchecked/topic/264894/Hida-Range

3)http://www.britannica.com/EBchecked/topic/599127/Tone-River, http://www.britannica.com/EBchecked/topic/311537/Kanto-Plain

Picture sources:

1) http://upload.wikimedia.org/wikipedia/commons/b/bb/Mt.Kobushigatake_from_Mt.Tokusa_03.jpg

2) http://upload.wikimedia.org/wikipedia/commons/4/4c/Mount_Shirouma_from_Mount_Korenge_2000-7-31.jpg

3) http://i.ytimg.com/vi/twF-puBoARc/0.jpg

4) http://en.wikipedia.org/wiki/File:Tone_riverine_system_20century.png