因此，他们的第一个日历系统被自然地分为三个季节，每个季节都是根据河流的变化而划分：淹没期、下沉期和丰水期。
So, naturally, their first calendar was divided up into three seasons, each based on the river's changes: inundation, subsidence and harvest.
丰水期水质比较稳定，枯水期水质变化相对比较大。
The water quality was more stable in high flow period and more changeable in low flow.
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