My guess would be before the crossing during the training of all units involved in the crossing since the actual crossing was at night/early morning.
Basic procedure of an assault river crossing is 1st an assault boat crossing of infantry followed imediately by infantry footbridges, then an infantry support bridge, then M2 Treadway bridges.
One account of the crossing can be read here:
TITLE:
An Engineer`s report of the Roer River Crossing at Linnich, Germany.
http://www.benning.a...rles R. LTC.pdf
The 84th Inf Div of XIII Corps crossed the Roer at Linnich, crossing began at 0330 on the morning of 23 February 1945. The 309th engineers had the responsiblity of assault boat crossing. The 1149th Engineer Combat Group was placed in general support of the 84th Infantry Division and the 171st Engineer Combat Battalion was placed
in direct support of the division with responsibility for the assault bridges. 74th Light Ponton Company and the
989th Treadway Bridge Company was assigned to the 171st Engineer Combat Battalion by the 1149th Engineer Combat Group for the operation.