the secon is from the ECEA series,delaware enduro "sluice" section,6mph section,basically ride down an extremely slippery concrete chute,ducking under the four or five beams running across the top,until it drops you into the river bed,all rocks and lots of flowing water,you cant see the holes(and there are plenty!!)and im sure if there wasnt any water it would be easier,i guess they run up the riverbed some years?
heres a helmet cam veiw first
http://youtu.be/mhjiQDysgd4
heres a different view
http://youtu.be/W91BguJO-Qo