North of Richmond and from five to seven miles distant the Chickahominy river runs in a course almost due east from its source. McClellan placed his main force north of that erratic and uncertain stream and there awaited the reinforcements for which he was clamorously calling. But he threw his left wing across the river to the Richmond side of it. Unless he were prepared to advance at once with all his force and assail the Confederate works this was an exceedingly dangerous thing to do, for the Chickahominy is a phenomenally uncertain and erratic river. In dry weather it is scarcely more than a brook, but in periods of rain--and spring in Virginia is a rainy season--it swells suddenly and quickly to almost impassable proportions, while the swamps that form its banks become morasses in which it is difficult to find even a foothold, and impossible to discover a fit camping place for troops. When McClellan established his left wing south of the river the stream presented no obstacle to its prompt reinforcement from the other side in case of need. But presently the windows of heaven were opened and the fountains of the great deep were broken up. The floods came, and this isolated left wing was cut off and left mainly to its own devices for self-maintenance. The Confederate General Johnston was quick to see and seize this opportunity. On the morning of the thirty-first of May, he assailed the detached left wing and there resulted the two-days' battle called Fair Oaks at the North, and Seven Pines at the South. Johnston's force scarcely, if at all, outnumbered the detached left wing of McClellan's army, but his hope was, by determined fighting to cut off that part of McClellan's army from the main body that lay north of the river, and to crush and destroy it before it could be reinforced. In his first assaults he was conspicuously successful, and had his expectation been realized that McClellan would be unable to reinforce his detached left wing from the other side of the river it is probable that Johnston's operation would have made prisoners of that wing of McClellan's army which lay south of the turbulent river. But two events stood in the way. One of the many frail bridges across the Chickahominy remained, in spite of the floods, as an available means of crossing. Some of its supports had given way under pressure of the waters and it was manifestly tottering to its fall. But General Sumner, ordered to support the imperiled force south of the river, heroically disregarded the danger and pushed his force across the frail and tottering structure, ordering his men to "break step" in the passage in order that the swing of the cadenced step might not cause the bridge to sway and fall. Thus perilously, he crossed, just in time to meet and defeat a Confederate effort to gain control of the bridge and destroy it, thus completely cutting off communication between the two wings of the Union Army.