1907 Lyon Gardiner Tyler
Page 46
”the King presently conducted me to another Kingdome, upon the top of the next northerly river, called Youghtanan.1…
1 Now known as Pamunkey River, which joins the Mattapony River at West Point, forty miles from Chesapeake Bay, to form the York River.”
Page 85
“Fourteene miles Northward from the river Powhatan, is the river Pamaunke, which is navigable 60 or 70 myles, but with Catches and small Barkes 30 or 40 myles farther. At the ordinary flowing of the salt water, it divideth it selfe into two gallant branches.1
On the South side inhabit the people of Youghtanund, who have about 60 men for warres. On the North branch Mattapament, who have 30 men. Where this river is divided, the Country. is called Pamaunke, and nourisheth neere 300 able men. …
1 Youghtamund (now called Pamunkey) and Mattapanient  (Mattapony).”
NARRATIVES OF EARLY VIRGINIA; 1606-16254
LYON GARDINER TYLER  1907
Lyon Tyler is quoting John Smith in this section. But what is important to notice is that John Smith was speaking of the Kingdom of Youghtanund, yet Tyler’s footnote references the name of the river. Clearly the rivers were named by the kingdoms living on them and the fact that the rivers name changed is a good indicator that the Indian kingdom that controlled the river changed. Combined with the fact that the Pamunkey were huge with 300 men and the Youghtanund were small with only 30 men, it seems safe to say they were either totally destroyed or had remnants absorbed into the Pamunkey, thus causing the river name to be changed.