Plan a canoe trip with portages.
Paddling time is distance over effective speed, where effective speed is your paddling pace plus or minus the current — and upstream travel can easily halve your progress. Portage time is the part that ruins schedules. A portage is not a single walk: you carry the canoe across, walk back empty, then carry the gear, so you cover the trail roughly three times at walking pace. A 400 metre portage is therefore 1.2 km of walking plus about ten minutes of loading and unloading at each end. Three of those add well over an hour to a day that looked like a straightforward paddle.
Paddling time
Hours = distance km / (paddle speed + current)
Portage time
Hours = (portages x length m x 3 / 1000) / 3 km/h + portages x 10 min
A time planning estimate only. Actual paddling times depend on wind, water level, rapids grading, load and fitness, and conditions can change quickly. Wear a buoyancy aid, check water levels and weather before departing, leave your route with someone, and do not attempt whitewater beyond your training.
Because a loaded canoe and a full set of gear cannot be carried in one trip. You portage the boat across, walk back empty, then bring the packs. That is three traverses of the trail, so a 400 metre portage means 1.2 km on foot — often over rough, root-bound ground.
Enormously, because it is additive to a modest speed. At a 5 km/h paddling pace, a 2 km/h current means 7 km/h downstream and 3 km/h upstream — the same distance takes more than twice as long against the flow. On river trips this dominates all other planning factors.