Final answer:
Flat-bottomed valleys between mountains are typically good places to find braided rivers. Braided rivers are defined by a network of river channels with sand deposits and gravel, which fits the given description of the environments better than a meandering river.
Step-by-step explanation:
Flat-bottomed valleys between mountains would be good places to look for braided rivers or streams. A braided river consists of a network of river channels that split and join, often characterized by sand deposits, sediment beds, and a layer of gravel that may feature cross-bedding—these are typical of environments where braided rivers are found. In contrast, a meandering river tends to scour the outer bend of the river channel and deposit sediment on the inner bend, which is not indicative of the flat-bottomed valley environment described.
Between the two environments described in the question, it is the one with mostly sand deposits and gravel that matches the characteristics of a braided river. If we were to map and locate features associated with a river system, in a braided river we could identify structures such as channel bars and multiple interlocking channels.