Final answer:
The 1,300-mile-long river that forms the border between Venezuela and Colombia is the Orinoco River.
Step-by-step explanation:
The river that forms part of the border between Venezuela and Colombia, and which is approximately 1,300 miles long, is the Orinoco River. It is one of the longest rivers in South America, streaming primarily through Venezuela.
The Orinoco and its numerous tributaries span a vast area and play a significant role in drainage, as well as being home to unique ecosystems. Despite the enormity of the Amazon River and its tributaries, which includes a drainage basin covering 40 percent of South America, the Orinoco River is the correct answer to this question, as it specifically borders Venezuela and Colombia.