No. Not reliably. But you only need to be wrong once.
Depending on how abrupt the drop is, you may only see a horizon line, with no white caps from rapids below. If there is enough wind to put riffles on the water, you may not notice it. An abrupt drop will have all the noise hidden below the horizon, so sound will be substantially attenuated.
Factors that increase risk:
- The current is faster than your back ferry.
- There is wind that masks the sound of the water.
- You are preoccupied, chatting, singing old songs from your youth, not paying attention.
- You are running without good maps.
- You haven't pre-marked the maps with potential trouble spots.
- You have no journals from previous trips.
- High volume rivers are more dangerous. Wider, deeper, faster.
Factors that decrease risk:
- You have compared the maps to google earth shots, and marked any white water that doesn't show on the maps.
- You have marked the maps up with notes from log books, journals etc. This can be problematic on long unused routes. E.g. The MacFarlane River, when we did it, we found: One account of the bottom quarter done in 20 years before; one account from the 1930's from a member of the Canadian Geological Survey of the middle half of the river. A mention in Tyrell's book that it was used as a aboriginal route to avoid going up the Athabasca. The upper quarter we had no data but the maps.
Several times, once on the Foster River, once on the Tazin River, once on the Taltson River I have found rapids that were not marked on the map. Certain things, however are triggers. These are not certain, but are "There may be dragons..."
- A contour line crosses the river.
- The river narrows to 1/3 of it's previous width.
- Both shores have rocky outcrops.
- There is a horizon line on the water -- you see tree tops -- not trunks -- beyond the line.
- The map changes from 2 parallel lines (showing channel width) to single line. (special case of the previous item)
- Anytime current is faster than your back ferry.
The Foster River one was the scariest. Decades ago, I don't remember what triggered me, but I called "Running order" (Canoes 50 m apart single file, experienced and less experienced crews alternating.) We crept along the side watching the shore line to gauge our speed vs effort. From 50 meters away it was obvious. We went to shore and scouted.
It was a class VI rapid. The kind that takes canoes and spits out fiberglass toothpicks. Not marked on the map.
Even doing all of the above is no guarantee. I have also found marked rapids that turned out to be nothing at all. When water levels change, some rapids are covered by water backed up at a narrows below, or revealed at lower flow rates. A rapid that is an easy Class II boulder garden at low water, can become III+ hair raising with boiling eddies and whirlpools with another 2 feet of water. Or the same water rise can turn an active rapid into moderate, dull fast water.
This makes wilderness tripping fun.