Best activity in summer (my favourite and maybe all year favourite). Enjoy walking through turquoise blue water and up green moss valleys. Swim through pools and climb waterfalls, fight the rapids. When it is 30-35C and humid... you are cool.
Usually about 3 hours as you can get very very cold.
Many routes near Tokyo you can get changed near the road at the lower starting point, leave bags in forest and sawanobori upstream. Walk back on road and pick up bags.
BUT
Even small gentle streams can be very DANGEROUS.
This information is from my personal experiences doing sawanobori. We learnt a lot and fortunately no accidents but the difference between a near miss and a serious accident or death is very small. You must respect the river and rocks when doing sawanobori.
Helmet (must wear one)
Even walking in 1cm of water you can slip and crack your head on rocks. It is too easy.
Breaking an arm or a leg, well that is a normal risk of sawanobori and you will get better. Breaking your head is different.
Also common to see falling rocks in stream valleys.
Rapids and fast water can push your head into rock sides etc. Slip climbing an easy slope and your head is in danger without a helmet.
Sometimes you need to climb around waterfalls and often loose rocks falling from people above. Same for entry and exit from streams up steep banks.
Watch Other People At All Times (everyone stays together)
Everyone is responsible for others in sawanobori - not just the leader. Water can kill in 1 minute.
You need to watch other participants at all times and NEVER let the group split up so people cannot be seen.
I personally had to jump down a waterfall and find someone underwater. The other people and the leader were walking away chatting and smiling up the stream for the next waterfall. They left a person who was still swimming through a small pool and just disappeared under the water. It was lucky me and a Japanese guy were watching from the side.
First, it was hard to find her under the water. It was hard to pull the woman up to breathe as rucksacks get heavy with water. Trying to hold her up also made it difficult for me to stay above water. She was lucky.
It was caused by the next danger point - COLD.
Cold
Everyone is a bit different but your body will start to lose a lot of core heat while doing sawanobori. It is easy to suffer from hypothermia. That is why most sawanobori events last a few hours or less.
Watch other people for shivering, lips face turning blue, slow reaction and lethargic. They can lose consciousness very quickly in cold water pools and drown.
The water may not be clear (especially after rain - maybe 2-3 days later) and you cannot see them underwater. They could die while you are chatting or looking for them.
This often happens when people have to wait while others climb waterfalls or swim through a pool and climb one at a time.
Walking Time
Sawanobori hiking is much slower than hiking on mountain paths. Perhaps 6 times longer on average but could be more if difficult and ropes used. A beginner sawanobori event in Okutama will takes maybe 3 hours to cover a hiking map time of 40 minutes along a forest road!
Ropes and Hand Signals
Ropes can be a danger in sawanobori and on waterfalls the noise will stop communication by voice. You need to plan beforehand what you will do if someone slips while climbing a waterfall.
You need a clear sight to the climber ( or intermediate person) and someone at the bottom watching. You will need a set of hand signals to control the climb, especially if there is a fall.
When someone slips the rope could hold someone at the bottom of the waterfall under heavy water spray. People cannot breathe and it is like waterboarding torture.
Even if a bit slack the rope could hold the person in eddy pool under the waterfall. and even the best swimmers will panic when they struggle to breathe.
Carry a small knife to cut the rope if needed and make sure everyone understands hand signals for taking in or releasing rope or freeing the rope.
I witnessed this at a waterfall with 5-10cm more water than usual due to heavy rain a few days earlier. 5cm extra in the river makes a big difference at the waterfall.
Much more powerful and noisy.
A woman slipped while climbing a 2m waterfall (only 2m) but you have to swim and start climbing. She was held by the rope under the waterfall circling.
The person controlling the rope could not hear us. She had worked as a lifesaver on a beach near Tokyo but was now panicking. I swam to help but she was hysterical and tried to climb on me. I had to swim away and around her back. I asked her to release the karabiner on the rope (she was a rock climber as well) but she could not do it.
As soon as the rope was released she floated away from the waterfall spray and was fine.
Simple tasks nay become very difficult under these conditions. If you think you can control the situation you might be surprised how difficult it can be - you may have difficulties yourself stuck in waterfall spray and cannot breathe, or people panicking when you try to help.
A knife is useful if you have to cut the rope quickly to release someone.
Clothing and Rucksacks.
NEVER WEAR COTTON.... it sticks to your body when wet and stops you moving freely. It also makes you colder as it does not insulate your body and hold water (makes you heavier too).
Stretch lycra polyester is good but basically only nylon / polyester clothing on your body.
Rainwear over clothing can help to keep you warmer but pockets and hoods can fill with water. Just be careful if you use them and do not put your valuables or wallet in a jacket pocket without a zip! It will be washed away. Best in a sealed water tight bag in a rucksack or dry bag.
Rucksacks hold water after they have been submerged and will feel very heavy. They can be a danger when swimming or climbing. Use a small one if no other options and make sure the top is covered to stop water spray entering.
Big packs are going to be a BIG problem. Ideally the dry bags are best. Fancy food that fills your pack is not a great idea if it is an overnight trip.
Food
You will need some high calorie snack to keep warm and if people look pale or blue then stop for a break and eat sugary snacks.
You will still need to carry water for a drink!
Getting In and Out of the Stream
In Japan you can find sawanobori grading for routes. But one thing people often overlook is the access. Often i did beginner sawanobori routes for OCJ but the hiking path into the stream or back out can be higher intermediate - steep slopes with no path. Do not forget this as it can be dangerous.
Mountain Weather and Thunderstorms
Might be a fine sunny day in Tokyo or at Okutama station but just 3-5km away there is rain and maybe lightning in the mountains.
Streams start to fill quickly when it rains suddenly up stream and the force of the water is much greater.
Also usually colder and dirtier colour.
The last place I want to be when there is a thunderstorm is standing in the water in a stream. Thunderstorms often start in the early afternoon so best to do sawanobori from around 9am and plan to finish by 1pm if there is a chance of thunderstorms or rain.
Be Careful Packing
It is easy to lose things in fast water and I have seen it happen several times.
Packing to Stay WaterproofOn a previous event a new leader was so happy with her new rope, but did not secure it properly on her rucksack. After walking through the first set of rapids the new rope was gone (never used).
Another guy put his wallet in triple plastic bags in his jacket pocket but the pocket did not close. It got washed away but very luckily I found it hours later floating in a pool at the bottom of the stream on the way home. Things in pockets without zips will be washed out and lost.
Footwear
More later