Meanders usually appear wherever a river goes down a gentle slope, flowing around obstructions, through fine-grained soil that easily erodes but sticks together well enough to make firm banks. Apparently the Goosenecks of the San Juan formed eons ago on such a flood plain. Then the land uplifted while the stream cut down to shape the 1500-foot chasm the Goosenecks course through now. A river bends as it adjusts to disturbances, such as, increases in water volume or obstacles that deflects its current. The greater the curve, the faster the water rounds the bend, takes off on a tangent across the river, collides against the opposite bank, and starts carving another bend.
Curves enlarge because of water dropping to the riverbed.
The dropping water deepens the channel on the outside of the bend. You asked a good question, Eloy.