As upstream waters meet where the slop flattens out, form a wider and sluggish body of water known as the middle river.Here, in the sunlight phytoplankton flourish and increase the productivity.

• 上游的支流汇集，进入中游，地势渐缓水量大增，日光直射在宽广河道上，富含营养物质的溪水缓流过鹅卵石底质。