深い and 浅い: depth, understanding, and state
Use 深い and 浅い for literal depth in water or holes and for degrees of understanding, relationships, sleep, and breathing. Check which scale the noun provides.
A long or short distance to the bottom
深い describes a large distance from a surface or opening to the bottom. この湖は岸の近くでも深いので注意してください。 warns that the lake has substantial physical depth even near the shore.
浅い describes a short distance to the bottom. この川は岸の近くなら浅いです。 and 穴を浅く掘りました。 compare the physical depth of water and a hole.
Depth of understanding and relationships
理解が深い means that someone understands a subject thoroughly and reaches beyond its surface. 先生は日本の歴史について深い知識を持っています。 applies depth to knowledge rather than space.
考えが浅い and 理解が浅い can judge that thought or understanding lacks sufficient depth. Because this may sound critical of a person, state what subject is being evaluated and use the expression with attention to the situation.
States with distinguishable levels
眠りが深い describes sleep from which someone does not wake easily, while 眠りが浅い describes light sleep. 昨夜は眠りが浅くて何度も目が覚めました。 says that the speaker repeatedly woke because the sleep was light.
呼吸が深い and 呼吸が浅い similarly describe the extent of breathing. Other scales may choose different adjectives: 濃い and 薄い are often more natural for dark or pale colors. Do not transfer English deep and shallow to every noun mechanically.
Quick check: If the bottom is visible close to the riverbank and the water is not deep, should you say 川が深い or 川が浅い?
Show the answer and why
Say 川が浅い. The sentence describes a short physical distance from the water's surface to the bottom.