It depends on the situation you are dealing with.

When you are talking to someone and the stream/river is in front of you and him/her, you use 這.e.g 這條河很清澈。

When you describe a river to someone you are speaking, you use 那. e.g. 那條河水很深。

It depends on the situation you are dealing with.

When you are talking to someone and the stream/river is in front of you and him/her, you use 這.e.g 這條河很清澈。

When you describe a river to someone you are speaking, you use 那. e.g. 那條河水很深。

Update: Common characters used before mass nouns of water:條, 片, 池, 場 e.g. 這條河, 這片大海, 這池死水, 這場大雨