Q. World's longest estuary is made by which river?
Answer: Ob River
Notes: The Gulf of Ob is the world's longest estuary. It's located in the Arctic Ocean at the mouth of the Ob River in northern Russia. The Gulf of Ob is roughly 1,000 km long and 50–80 km wide. It's also the migration ground for Beluga whales.  The Ob River is one of three great Siberian rivers that flow into the Arctic Ocean. The other two are the Yenisei River and the Lena River. The Ob River forms 25 km (16 mi) southwest of Biysk in Altai Krai at the confluence of the Biya and Katun rivers. The main city on its banks is Novosibirsk, the largest city in Siberia, and the third-largest city in Russia.