Simplify the following text for a Grade 10 native English-speaking person.
Original: The post developed successfully as the first navigable port on the Congo River above Livingstone Falls , a series of rapids over 300 kilometres ( 190 mi ) below Leopoldville .
Simplified: The post flourished as the first navigable port on the Congo River above Livingstone Falls , a series of rapids over 300 k below Leopoldville .