
Use that when the words following it are necessary to identify the word that refers to.
Example: "The river that flows by my door is rising."
You cannot remove the that clause, because you wouldn't know which river is being referred to. The sentence would simply read: "The river is rising."
Use which when the words following it are not necessary to identify the word it refers to.
Example: "The Indian River, which flows by my town, is rising."
You can remove the which clause, because you would know which river is being referred to - the Indian River. The sentence would read: "The Indian River is rising." Note: When using which, use commas to separate the clause. When using that, don't use commas.