Three Parallel Rivers National Park
Three rivers — all with headwaters in the Qinghaai/Tibet plateau — run roughly parallel here for 170 km (just over 100 miles). Downstream they diverge, to become:
They have different names here. The Salween is Nu Jiang, the Mekong is Nan Sha Jiang, and the Yangtze is Jin Sha Jiang.
Their mouths are thousands of kilometers apart, flowing into different oceans. Their headwaters are also fairly widely separated. However, in this region they are quite near each other, flowing along three enormous more-or-less parallel gorges which wind through a region of large mountains.
These are large rivers. All three are on Wikipedia's list of the world's largest rivers . Here is a summary, comparing them with other well-known rivers:
India and the rest of Asia are on separate continental plates which are colliding; the impact creates the world's largest mountains. This region has over 100 peaks above 5,000 meters and some over 6,000. For comparison, Mont Blanc (the tallest mountain in Western Europe) is 4,810 and Mount Whitney (highest point in the 48 contiguous United States) 4,421.
The area also has gorges several thousand meters deep as the rivers cut through these mountains.
Flora and fauna
The UNESCO citation  adding this region to the World Heritage List describes it as "an epicentre of Chinese biodiversity ... one of the richest temperate regions of the world in terms of biodiversity".
The climate is also very diverse. The latitude is almost tropical, but the region is at quite a high altitude, so the basic climate is temperate. However, with the complex structure of mountains and valleys, there is a huge range of micro-climates. In winter, much of the area can get quite cold, especially at higher altitudes.
The population is also quite diverse. China has 56 officially recognised ethnic groups. About half of those can be found in Yunnan, and several in this region.
There is some risk of altitude sickness.
More generally, the area is remote, mountainous and not much touristed; be certain your health, skills, and equipment are adequate for the conditions. Consider taking an organised tour or hiring a guide.