Painting above: Yumas. In: “United States and Mexican Boundary Survey. Report of William H. Emory…” Washington. 1857. Volume I.
In 1871, a document titled “Resources of Arizona Territory with a Description of the Indian Tribes; Ancient Ruins, Cochise, Apache Chief; Antonio, Pima Chief; Stage and Wagon Roads; Trade and Commerce, Etc.” was published by the authority of the Legislature. In a section titled “Indians of Arizona,” it tells us the following:
The Yumas and Mohaves live along the Colorado River, are and have been for some time at peace with the whites and have received the largest portion of the appropriation made annually for the benefit of the friendly Indians of this Territory. They have received just enough assistance from the government to cause them to believe that they can live without work, but too little to keep off the gnawings of hunger. The results is that they are reduced to the lowest stages of degradation. Licentiousness and disease are doing their work, and soon these Indians will pass away from the face of the earth.
Charley-Arri-Wa-Wa (Mohave), 1872
Note: Today the Yuma are known as the Quechan people.
Native Heritage Project 
Hardly fallen off the face of the Earth. I had the honor to do security and live at Ward Valley in 1998, where we won in a stand off against the Federal government at the request of the Ft Mohave Nation. This small 1,600-member tribe retains its vitality and culture in the face of great odds
It’s very interesting to see the difference of an 1871 perspective vs what actually happened.
Yuma are also known as Yuman, Kwtsan, Kwtsaan and Quechan.
Their official page says they prefer to be called Quechan.
http://itcaonline.com/?page_id=1173
Wikipedia says:
Estimates for the pre-contact populations of most native groups in California have varied substantially (see population of Native California). Alfred L. Kroeber (1925:883) put the 1770 population of the Quechan at 2,500. Jack D. Forbes (1965:341-343) compiled historical estimates and suggested that before they were first contacted, the Quechan had numbered 4,000 or a few more.
Kroeber estimated the population of the Quechan in 1910 as 750. By 1950, there were reported to be just under 1,000 Quechan living on the reservation and another 1,100+ off it (Forbes 1965:343). The 2000 census reported a resident population of 2,376 persons on the Fort Yuma Indian Reservation, only 56.8 percent of whom said they were of solely Native American heritage. More than 27 percent identified as white.
http://nativeamericanencyclopedia.com/quechan-indian-tribe-of-arizona/