After the last ice age, post-glacial rebound has changed the topography of Finland enough to cause some lakes to develop new channels and to discharge into different directions than before. The rising surface level of Lake Näsijärvi to the north of Tampere eroded the sandy isthmus between Lake Näsijärvi and Lake Pyhäjärvi sufficiently to break a new channel into it about 7500 years ago. That channel is known as Tammerkoski today.
Here's a picture of Finland immediately after the last ice age:
