Complex sentence: The French had established a trading post at the island Saint-Louis in the mouth of the Senegal river in the 17th century, and Bedouins of Mauritania came to control much of the trade which reach the French from the interior.
Split into simpler sentences: The French had established a trading post at the island Saint-Louis in the mouth of the Senegal River in the 17th century. The Bedouins of Mauritania came to control much of the trade that reached the French post from the interior.