The characters combine to form a potent metaphor: “The great rivers (江河) are declining day by day (日下).” Just as the shrinking of a nation's lifeblood rivers would be a disaster, this idiom describes a serious and continuous deterioration.
The power of 江河日下 comes from its deep roots in Chinese geography and history. The Yangtze (江) and Yellow (河) rivers are not just bodies of water; they are foundational symbols of the nation itself. For millennia, their flow meant prosperity, and their flooding meant disaster. The idea of them “declining daily” evokes a sense of fundamental crisis and loss. In Western culture, the closest equivalent might be “going downhill” or “on the decline.” However, these phrases are often more casual and can apply to smaller-scale personal issues. 江河日下 carries a more literary, formal, and epic connotation. You wouldn't typically say your mood is 江河日下, but you absolutely would to describe a nation's economy, a traditional art form's popularity, or the moral fabric of society. It implies a fall from a past golden age, a common theme in Chinese historical and philosophical thought. It taps into a collective cultural anxiety about decline and the loss of greatness.
江河日下 is a formal and literary idiom, making it more common in writing (news reports, academic essays, formal speeches) than in casual conversation. It always has a negative connotation, expressing concern or criticism about a deteriorating situation.