To find the answers we will have to define the term altitude first.

Intermediate altitude 1500m-2500m
High altitude 2500m-3500m
Very high altitude 3500m-5800m
Extreme altitude >5800m

The specifications are made from medical surveillance and the oxygen saturation level in the blood.

Most people will never really venture above 2500m and might experience light altitude sickness. The usual danger zone is in the region between 3000m-4500m as usually ascent into those heights (in the Himalayas, Karakoram and Andes) is decently easy and involves the usual trekking routes.

Obviously there are various prevention methods with the slow ascent once above 3000m the most important one.

The aspects of altitude sicknesses (AMS, HAPE, HACE) are not the issue here but knowing the dangers and symptoms are important once you plan to ascent higher then 2500m.

Kayaking obviously is a physical sport in low altitude already and obviously, once advancing in heights of 4000m+ the aspect of lesser oxygen saturation is an issue by attempting difficult rivers.

Rivers are graded between I-VI with difficult or expert rivers are starting from IV-V with VI the exemption of the navigable possibilities.

I have traveled and kayaked in altitudes up to 5600m and never really experienced serious problems in altitude.

Carrying weight (loaded boats can weigh up to 50kgs) and doing physical exercise starts to affect a trained person at an altitude of 4000m+ when arterial oxygen saturation will be below 90% and it takes the extra effort to make the necessary paddle stroke or while doing a portage along a cliff.

But fitness is definitely not prevention for altitude sickness, as most people in high grounds are still climbers, skiers or other athletes usually fitter then the average person still come down with variations of AMS.

Slow acclimatization is the key to avoid problems in altitude and descent to lower regions the only real treatment (apart from pressure chambers) if any symptoms occur.

With many rivers in the Himalayas and Karakorams starting at altitudes between 2500 and 4000m and or passes to be crossed for access, it is important to not underestimate the impact of AMS and to be aware of the symptoms. Exhaustion and, in altitudes above 4000m, also mental exhaustion can and will be an issue for the success of an expedition in high altitude kayaking.

There are only a few rivers in the world which will still be navigable once above 4500m. The usual build up of Mountain Rivers in the Himalayas will give you good waters at an altitude of about 3500m-4500m and then usually descent into deep boulder choked gorges formed by the still young and changing mountains there. The steepest part of the rivers will be over at roughly 2500m with challenging rapids all the way into the Ganges plains with only 80m above sealevel.

Recommended places to try kayaking in altitude are the Tibetan plateau (now just opening up to commercial trips), the Zanskar and Tsarap Chu Rivers in the Ladakh area (Indien part of the Tibetan Plateau), the Indus , Braldu and other Karakoram Rivers in Pakistan.