This article is from the Holocaust FAQ, by Ken McVay email@example.com with numerous contributions by others.
There are pictures of these ditches with corpses burning in them
(See pub/holocaust/gifs for available photographs) . During
the period in which Auschwitz was active, the SS had thearea
drained; as can be seen today, the drainage, which has not been
maintained since 1945, has deteriorated and the water level had
risen. Furthermore, the ditches were not used all the time, but only
when there were not enough furnaces to cremate the victims or during
the extermination of the Hungarian Jews, when so many people were
murdered daily that the furnaces could not handle the amount of
corpses. The "incineration ditches" are known to have been active in
October-November 1942 and the summer of 1944, which do not coincide
with the period the snow melts and the water level rises.