Explanation:

 The width of continental shelves largely de­pends on the nature of reliefs of the coastal land i.e.:

(1) the shelves are narrow where high mountains are very close and parallel to the coast (e.g., the Pacific conti­nental shelf along the western coast of S. America is narrow (16 km) because of the presence of the Andes mountain), and

(2) the shelves are wider where the coast lands are wide plains. Though the continental shelves are generally wider in front of river mouths but the shelf off the Mississippi mouth is exceptionally narrow.

For concept clarity please visit following link:  http://www.geographynotes.com/oceanography/relief-zones-of-ocean-basins-oceanography-geography/2566