In order to elucidate factors controlled the water column stratification in canyon-reservoirs, and describe the relative response of water quality, we present a 13-month survey and study of meteorological, physical and hydro chemical patterns during 2001 in a canyon-reservoir, southwestern China, especially including a intensive frequency study during the period from August to Novemher of this year in which an event of water quality abraptly degeneration occuired.The study showed that: an obscure thermal stratification has been formed on vertical profile of water column during summer (about from April to October) in Baihua Lake reservoir, however, it could cause the very strong hydro chemical stratification (i.e.dissolved oxygen stratification) in same period in water column of the reservoir.During summer of 2001, the oxide-reduce interface could move up to 8 meter-depth from bottom of Baihua Lake reservoir.The survey results also confirmed that gravitationally unstable of the thermal stratification caused by surface cooling of water when the air temperature decreased abruptly in early fall, leaded to a vertical convection between the upper layer water and deep layer water.The reductants (e.g.Mn²⁺, Fe²⁺ and H₂S) formed in the anoxic hypolimnion companying with processes of organic maters degeneration would brought to the epilimnion through the vertical eddy diffusion.These reductive matters would consume up the dissolved oxygen in the utter water rapidly as if such oxidization reactions were strong enough, and than a large number of aquatic organism dead due to oxygen deficient.This study showed that the seasonal thermal stratification, although not strong like typical alpine lakes, could play a key role in change of water quality in canyon-reservoirs.