HEADQUARTERS DEPARTMENT OF ARKANSAS, In the Field, Camden, April 17, 1864. (Received 9.06 p. m., 25th.)

GENERAL: The troops from Fort Smith were to have joined me at Arkadelphia. On the 1st instant Thayer changed his route, and all my messengers failed to communicate with him until the 5th, at Rockport. I had crossed the Little Missouri at Elkin's Ferry, and was on high ground, having found the other roads to Camden impassable. A heavy rain fell, raising the Little Missouri so that it could not be forded, and rendered it necessary to corduroy about 5 miles of bottom and lay the pontoon bridges to get Thayer across. He delayed us eight days, which he explains. We were bush whacked, and attacked in front, rear, and flank by Price's cavalry and twelve pieces of artillery. At the Little Missouri they had a line of breastworks crowning the hills, where bottom terminates, over a mile in extent. After a severe skirmish they fell back to Priarie D'Ane, where another severe skirmish took place. The rebels were always repulsed with considerable loss on our part and with greater on theirs. The rebels fell back across the prairie to a line of rifle-pits and epaulements for guns en barbette along a skirt of timber, commanding the Washington and Camden road and the approach on the prairie for one-half a mile across. This part of the prairie and the works were hid from our view by a belt of timber along a creek. We developed their position by skirmishes, and turned their left flank, and would have had an enfilading fire, but Price, who commanded in person, withdrew his forces and retreated toward Washington. We pursued a few miles, and then turned toward Camden. Our supplies were nearly exhausted, and so was the country. We were obliged to forage from 5 to 15 miles on either side of the road to keep our stock alive. My spies sent to Banks did not return. Those he promised to send did not reach me. Everybody said Banks had been repulsed below Natchitoches, and had fallen back. Telegraphic dispatches have been received at this place to the same effect. One of my spies sent to Banks has come in, and confirms the reports. He says he went to within 30 miles of Alexandria. The rebels had endeavored to destroy all produce that they could not consume or carry away for considerable distance around this place. When they found we had turned this way they tried to beat us here. Marmaduke got in our front and Dockery in our rear, by the middle and north roads, and endeavored to hold us until price could get into the fortifications by the south road with his infantry and artillery, having evacuated Camden under the supposition that we were marching on Shreveport, by the way of Washington. We marched 23 miles the last day into Camden, driving Marmaduke before us from position to position. Cooper and Maxey have come from the Indian Territory, and Price is now concentrating at Woodlawn. An immense amount of labor has been expended in fortifying Camden and cutting away forests. There are nine forts on eminences, and they seem to be well located. Strategically and commercially, I regard this as the first town in Arkansas. The Washita is navigable always for several months in the year to this place, and sometimes to Arkadelphia. At present there is 24 feet in the channel. We captured a large steamer (the Homer) 30 miles below here, which [the enemy] left on our approach. With a gun-boat as convoy we can get supplies up this river. I expect supplies soon from Pine Bluff,