Is the St. JOhns river a mature one or a young one?

Question

im doing a reaserch paper, and it would be useful if there is any other information you know on the river

Answer 1

More than 100,000 years ago, the area of land that now comprises the river was connected to the Atlantic Ocean for most, if not all, of its length, making the river nothing more than an extended system of lagoons and tributaries. As the ocean levels dropped, barrier islands and reef formations effectively walled off the system of lagoons from the ocean, forming the river.

The Mississippi is considered a "mature" river at 75,000 years old. The St. Johns is not a youthful river.

Answer 2

The elevation change from headwaters to mouth is only about 30 feet, making the St. Johns one of the world's "laziest" rivers[1]. Its extremely low velocity, combined with the generally level elevation, causes the St. Johns to spread out to a great width for much of its course.

Youthful river – a river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider. (Examples: Brazos River, Trinity River, Ebro River)
Mature river – a river with a gradient that is less steep than those of youthful rivers and flows more slowly than youthful rivers. A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper. (Examples: Mississippi River, St. Lawrence River, Danube River, Ohio River, River Thames)
Old river – a river with a low gradient and low erosive energy. Old rivers are characterized by flood plains. (Examples: Huang He River, Ganges River, Tigris, Euphrates River, Indus River, Nile River)
Rejuvenated river – a river with a gradient that is raised by tectonic uplift.