because of the low level of the fens landscape, the canal was constructed on embankments for most of its 5.25 mile (8.4 km) length, and was opened in 1797. flood locks were constructed at both ends of the canal. the one at outwell was long, but the one at wisbech was only , and so longer boats wishing to enter or leave the canal at the wisbech end had to wait for the levels in the river and the canal to equalise, at which point both sets of gates could be opened. the canal did not have its own water supply, but was refilled when the water level in the nene at wisbech was high enough. this was normally only at spring tides, and so the water level often ran low in the period before the next spring tide was due.