It seems that you developed irritant contact dermatitis
on knee...due to some irritant while going to river .
And the lesions complicated with bacterial infection.
The lesions became reddish and filled with pus.
Avoid to walk, take bed rest
Apply antibiotic cream. Cleanse the area with antiseptic.
Take a short course of antibiotics and anti inflammatory drugs.
It may heal in few weeks.
Take immunobusters and healthy food.
Eventhough, you consult dermatologist
I hope you got my answer.
Dr. Ilyas Patel MD