More than a hundred animals, including 10 one-horn rhinoceroses, have died due to the deluge at the Kaziranga reserve in north-eastern India since June.
Park director P Sivakumar said: “Since the first week of June, we are having no respite with wave after wave of flood that has wreaked havoc inside the Kaziranga National Park and tiger reserve.”
He said an animal that drowned in a swollen river near the park on Saturday brought the rhinoceros death toll up to 10.
William and Kate visited the park in April 2016 to learn about conservation and anti-poaching efforts.
In their letter to Mr Sivakumar, they wrote: “The deaths of so many animals, including one-horned rhino, are deeply upsetting.”
The ongoing monsoon has dumped rain across parts of India, Bangladesh and Nepal, displacing 9.6 million people, according to the International Federation of Red Cross and Red Crescent Societies (IFRC).
More than 550 people have been killed in the floods, the IFRC said.
The organisation warned of a humanitarian crisis, saying close to one third of Bangladesh has already been flooded, with more rain expected in the coming weeks. It said 2.8 million people have been affected, and more than one million are isolated.
In India, more than 6.8 million people have been affected by the flooding, mainly in the northern states of Assam, West Bengal, Bihar and Meghalaya bordering Bangladesh, the IFRC said.
In Bihar, officials say at least 10 people have been killed.
In Assam, home to Kaziranga, 96 people have been killed in floods and another 26 have died in mudslides.
The floodwaters are making protecting wildlife within the park more difficult, too.
Mr Sivakumar added: “More than a hundred of the 223 security camps inside the sprawling park are still submerged, making day-to-day work of our 1,600 guards really challenging.”
