regarding a fence, it appears that livestock use the rio grande for water and a fence would make that difficult.
from here
http://coloradoriparian.org/GreenLine/V09-4/RioGrande.html
The river is the primary source of water for livestock on both banks. In this desert country, cattle are attracted to the succulent vegetation of the riparian area, where they remain until moved or the forage is used up.
On the BLM lands, livestock access to the river is restricted by a fence between the riparian area and the uplands. Small fenced water gaps allow livestock access to the river to drink.
Pure BS, all across the country the department of agriculture has been/and are paying farmers to fence their cattle off from water and to put in pumping systems with water tanks.