The mouth of a river is where it lets out into the sea, and that’s the same in both English and Japanese. (The word is 河口 かこう・かわぐち or 川口 かわぐち.) The word for a river source actually doesn’t use a kanji for river at all: 源頭 げんとう (source; origin + head).
You were close with 代肉. The word 代用肉 だいようにく actually exists, and it refers to either meat substitute or cheaper meat used in place of beef or pork! There’s also 人工肉 じんこうにく (man-made meat) for the first sense.