In all provinces of Tamriel, I see many rivers that seem to flow northward (or somehow magically spring from nothing and flow south) , most notably the Niben and it's various branches, and Hammerfell's river system. Yet at the same time there are rivers which flow south, mainly in Valenwood and (most noticeably) the Black Marsh.

Is there any mention of northward flowing rivers in books, or am I simply noticing an unmentioned phenomenon?

Seems to be more the result of the north of Tamriel being more mountainous. Specifically Niben Basin is a rain forest so the water has to go somewhere and it can't go up. Similar arguments can be made for the other rivers.

IRL as defined by wiki:
Some people think that most rivers flow from north to south.[6][7] Rivers in fact flow downhill regardless of direction, often in a complex meandering path involving all directions of the compass.[8][9][10]

Few major rivers in the continental U.S. flow north since most of the country is located in the watershed of the Pacific or Atlantic oceans or the Gulf of Mexico, with very few rivers flowing northward toward the Arctic Ocean, Great Lakes, or Hudson Bay. However, thousands of north-flowing rivers exist elsewhere, including such major watercourses as the Nile, Mackenzie, Rhine, Yenisei, Nelson, and Lena. Four of the ten longest river systems of the world flow mainly north.

I know that, but I was asking whether there is any confirmation as to whether the previously mentioned rivers actually do flow north or whether they are generated from small streams and such and flow south.

Doesn't Nabia go south?

lol Nibenay is Kyne's tissue.

I took the whole waterway issue as just a lack of effort in the TES series and never went so far as to wonder like you have. Now that you've brought it up it seems like those rivers in western Cyrodiil and southern Hammerfell all flow south west.

Well it explains the green snot that we're looking at in Cyrodiil.

Looking at the naming scheme with the upper Niben in the north, the lower Niben in the south, I'd say the Niben flows south into Topal bay :unsure:

The http://en.wikipedia.org/wiki/Monongahela_River (one of the three main rivers near Pittsburgh, Pennsylvania) flows north.

Indeed, and judging from the geography of it, that seems to make sense, of course this is not reflected in the game, but that's just because aside from water falls and the like (Which are, in terms of how they are done in game, not actually water.) Oblivion doesn't model flowing water. For rivers that do flow north, as in the information given from the Wikipedia quote in this thread, rivers always flow downhill, when that is north, they will naturally flow in that direction.

As for rivers seeming to spring from nowhere, that, I believe, is owed to unrealism of the game's geography rather than anything in lore.

The Freaking Nile River flows north.

My entire country, The Netherlands, is basicly a delta of mostly North-West flowing rivers.

I am ... speechless.

Looking at Cyrodiil's map, you see that the Nibenay basin is surrounded by mountains or hills: The Weast Weald, Colovian Highlands, the Jeralls and the Valus Mts. Rivers and streams that spring from those mountains naturally flow downward into the Nibenay basin (which is sometimes, e.g. in the case of the White Rose River, northwards). The lowest parts of the basin form Lake Rumare around the Imperial City, and from there the water flows the only way it can, to the south, as only there the terrain is low enough, and in the form of the Niben river carves its way down until it reaches sea level in the Topal Bay.

In the case of Skyrim (as most obvious case), the mountains (e.g. the Jeralls) are along the southern border, the sea is to the north. That's why its rivers usually flow northwards.

I can't see why the Niben appears to flow northward...it's source is Lake Rumare and the various feeder rivers which flow into the Nibenay Basin, from Oblivion and it's map. The whole thing flows to the sea in the south.
OR if you were meaning it appears to flow from nothing, I don't see how that is the case either, considering Lake Rumare, as it's source, is fed from various small rivers flowing out of the hills south of Pell's Gate, and presumably streams coming down from the Jeralls.