Thanks Stephen. "Washita" is the correct name of the town there -- I erroneously said "Wichita" in my earlier post.

The 'agency' seems to refer to the site of three buildings -- Leeper's house, Jones' house, and a store -- and not one "office building". Per the sources I have found, it seems the 'agency' is along Leeper Creek for a mile or two above the confluence with Lost Creek. However, Horace Jones said his house was four miles from the Washita River which would be a couple of miles north of the dam of the reservoir, assuming it was on upper Leeper Creek and depending on what point on the bend of the river he was measuring to.

Everyone knows I like maps so I have included a couple below. The annotations are mine. The one on the left is from a county township map. The one on the right is from my Mapsource software (for my GPS receiver). It is my guess that the reservior changed Leeper Creek -- splitting it into two -- since the creek that feeds the reservior is upper Leeper Creek.

Jones refers to the attackers on the agency and the Tonkawas as 'Osages' though records from the Indian Office in Kansas don't identify any Osage tribal members participated. I believe I have seen 'Osage' used as a more generic term applied to Indians who were considered 'wild Indians'.

Some sources on the location:

‘Civil War Sites in Oklahoma’, Muriel Wright & LeRoy Fischer, OHS 1967, p.9

NW1/4 Sec 2, T7N R11W and SW1/2 Sec. 35, T18N [T8N], R11W about two miles northeast of Washita, and aobut four miles northeast of the present town of Fort Cobb, on the west side of the Washita River. The Oklahoma Historical Society marker is located on U.S. Highway 62 and State Highway 9 about eight miles west of Anadarko at the junction with the county road leading north to the town of Washita, and about five and a half miles south of the site itself… The only indication of the original site is a grove of chinaberry trees in a fenced pasture SW1/4 Sec. 35, T18N [T8N], R11W.

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Chronicles of Oklahoma, Volume 16, No. 4, December, 1938, p.413
Historic Sites Around Anadarko, By C. Ross Hume
The permanent site of the first agency, selected near the bank of Leeper Creek, was not far from its confluence with the Washita River, where it continued to be maintained under Confederate auspices for a year and a half after its abandonment by the Federal Indian Service. Matthew Leeper had been appointed to fill a vacancy as agent of the Upper Brazos Reserve, a few months before the removal to the Washita, and was retained in the service at the time of the removal and was continued in charge after the Confederate authorities took over the Agency. After the withdrawal of the Federal garrison from Fort Cobb, most of the Indians at the Wichita Agency, frightened and confused by the outbreak of the war, and very suspicious of the significance of the Confederate movement, possibly because of their own exile from Texas less than two years before, abandoned their homes, and their little fields, and fled in the wake of the retreating Federal troops, most of them seeking refuge in Kansas, until after the War had ended. The Tonkawas and the Peneteka, or Honey-Eater Comanche band, together with the White Bead Caddos, remained on the Washita throughout that period, however.

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Chronicles of Oklahoma, Volume 2, No. 4, December, 1924, p.383-384
Horace P. Jones, Scout And Interpreter By Joseph B. Thoburn

I was living at the Indian Agency, about four miles north of the Washita… I and a friend were living in a log house of two rooms with a passage way between them. There was a store with two clerks in it, near by, and the agent’s home, where he lived with some of the employes. … Doctor Sturm was at the Caddo camp, three miles away, and I decided to foot it up there and warn him. The Indians were now attacking the trader’s store and the agent’s house, so there was no use of my going there. It did not take me long to find Doctor Sturm. He would not believe me when I told him the Osages were attacking the Agency, as he had great faith in the Caddoes and believed that they would have warned him. Going up on a hill, I showed him the Agency buildings on fire and he soon had no more desire to tarry than I had. He had only one horse, so we decided to take turn about riding it and go and warn a man named Chandler, who lived twenty-five miles south, on a creek which is now known as Chandler Creek, about ten miles from the present site of Fort Sill. We had by no means an easy ride, as we had no saddle and the pony was far from fat, but, as long as we could save our scalps, we did not mind losing a little skin here and there.

We arrived at Chandler’s just as the sun was rising. He saw us coming and guessed what was up. His family soon gathered around and we held a council of war. It was decided to get some breakfast, pack some grub, strike out for Red River and cross into Texas. I took a little nap while preparations for the journey were being made. We all got a mount and, with our provisions tied on our saddles, journeyed toward the Red River as fast as we could go. Red River was about fifty miles south of Chandler’s and we reached it late that night. We rested a while and then went on to the settlements—fearful all the time that the Osages were close behind us.

We sent word to some troops that were camped not far away and they started up to drive back the Osages. I went with them. On our return to the Agency, we found our worst fears realized. All of the white men had been killed and horribly mutilated and the Tonkawas were almost wiped out of existence.

It seems that the Osages had divided themselves into three bands. One of these had attacked the store, another had attacked the agent’s house while the other had come to my house. After finishing up the whites, the three bands combined and attacked the Tonkawas. The Osages were well mounted and had plenty of ammunition, while the Tonkawas had very few guns, being armed chiefly with bows and arrows. The Tonkawas were camped along the Washita River and, as they were attacked by the Osages, they fled toward the Keechi Hills along a stream which has since been known as Tonkawa Creek. They made little or no resistance and were butchered and scalped as fast as they were overtaken, women and children as well as men. About eight hundred were massacred and the tribe was practically wiped out

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