The Union Army straddled the rain-swollen Chickahominy River. Four of the Army's five corps were arrayed in a semicircular line south of the river. The V Corps under Brig. Gen. Porter was north of the river near Mechanicsville in an L-shaped line running north–south behind Beaver Dam Creek and southeast along the Chickahominy. Lee moved most of his army north of the Chickahominy to attack the Union north flank. He left only two divisions (under Maj. Gens. Benjamin Huger and John B. Magruder) to face the Union main body. This concentrated about 65,000 troops against 30,000, leaving only 25,000 to protect Richmond against the other 60,000 men of the Union army. It was a risky plan that required careful execution, but Lee knew that he could not win in a battle of attrition or siege against the Union army. The Confederate cavalry under Brig. Gen. J.E.B. Stuart had reconnoitered Porter's right flank as part of a daring circumnavigation of the entire Union army from June 12 to June 15 and found it vulnerable. Stuart's forces burned a couple of Union supply ships and was able to report much of McClellan's army's strength and position to Gen. Lee. McClellan was aware of Jackson's arrival and presence at Ashland Station, but did nothing to reinforce Porter's vulnerable corps north of the river.