As distinct as the Jīng and Wèi [rivers]
Indicates when things have very clear boundaries or are very different
Neutral
在历史上，这两种类型的电影可谓是泾渭分明 | Zài lìshǐ shàng, zhè liǎng zhǒng lèixíng de diànyǐng kěwèi shì jīngwèifēnmíng
Historically, these two film genres have been completely unrelated
[Source]
比较新的有关作个男人的定义远远没有那么僵硬和泾渭分明 | Bǐjiào xīn de yǒuguān zuò gè nánrén de dìngyì yuǎn yuǎn méiyǒu nàme jiāngyìng hé jīngwèifēnmíng
The new definitions for what it is to be a man are not nearly as rigid and clear-cut as they used to be
[Source]
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Pictures of where the Jing River and the Wei River meet are pretty amazing. I found one Twitter post with a good example here
《诗经》(Shī Jīng), or "The Book of Songs." This Spring and Autumn Period (770 - 476 BC) collection of poems is considered one of the five Confucian classics
This chengyu doesn't come from a story, but rather from the appearance of two rivers in China: the 泾河 (Jīng Hé) and the 渭河 (Wèi Hé). The 泾河 starts in the province of 宁夏 (Níng Xià) and flows southeastwards until its clear waters drain into the 渭河. The 渭河, the largest tributary of the 黄河 (Huáng Hé, Yellow River), has dark and muddy waters. Where the two rivers meet, the stark difference between their water colors becomes very apparent, and this eventually gave rise to the expression 泾渭分明