Hi, 시냇가 refers to the "stream" (and its surrounding area) and 시냇물 specifically refers to the *water* of the stream. So, "시냇가에 이르렀어요" means, "I/we have reached (arrived) the stream" and "이 시냇물이 얼마나 깊은지 아니?" means "do you know (or 'do you realize') how deep the water (of the stream) is?"