Around 100 dead hippos have been found covered in unexplained marks and bruises.
Heartbreaking images show the animals' corpses floating on their backs with their feet sticking out of the water.
The cause of the deaths is a mystery, although experts in Namibia suspect an outbreak of the bacterial disease, anthrax, which can occur in the arid climate.
Conservation volunteer Cathy Withers-Clark, 50, from Simon's Town, South Africa, took the photos in Bwabwata National Park while on a road trip around the African country.
She said: "I received an alert previous to seeing this that around a hundred hippos had been found dead in Bwabwat in the previous week and that anthrax was suspected.
"But with the park being very large, I didn't realise that I was right in the middle of the affected area.
"Me and my partner were spending a couple of days in a camp with a treehouse over the Okavango River in the Mahango Core area, and there were hippos happily 'huffing' to each other all the time right next to us.
"When we woke up a few mornings after, there was a dead hippo in the river in front of the camp, and the others had all moved further away from us, but I just assumed it was a one-off.
"We left camp and entered the part of the park closest to us and soon found that it was far from being a one-off - we were right in the middle of the outbreak.
"We saw vultures circling at first, signalling a predator kill we thought, as we drove in that direction to have a look.
"Soon we came to the riverbank, where we saw more than 50 dead hippos, some of which had clearly been dead for days.
"The area was full of vultures, marabou storks and other scavengers hoping for an easy meal, and there was no shortage for them.
"It was a devastating sight, especially as there were hippos that were still alive but could do nothing to assist their family.
"While we were there a group of scientists arrived to do more tests on the freshest bodies but we didn't want to disturb them with questions, so it was difficult for us to find out the cause of the deaths."
