I am looking for a map that shows European rivers.  I don't want the
map to include country borders, cities, or anything else.  The less
annotation the better.  Ideally, it would just show geography:
mountians, rivers, forests, etc.

I want a physical, printed map I can purchase.

Hello dellisny-ga:

It was a pleasure to answer your question. There are plenty of
websites that cater to huge number of maps but only a selected few
offer the type of map you are looking for.

The best of the websites is Geo S. mile. 
http://www.geosmile.com
Once on the main page, please select from the left hand side menu. You
can either select the ‘Physical maps’ or ‘Landscapes’. Both of these
categories have the kind of map that you are looking for. You may also
enlarge the map to see what it will look like once printed. There are
also some other maps that might be of your interest.

You may also want to look at the maps published by The Inland Water
ways Association at https://secure.iwashop.com/SHOP/Worldwide%20Waterways.html

There are some well known companies that publish the inland waterway
map of Europe. These are detailed maps of the European rivers.
Although you cannot preview the maps but again they are all high
quality and detailed.

MAPS WORLDWIDE
http://www.mapsworldwide.com/mwwlive/m10924.htm

GOLDEN ED
http://www.goldened.com/store/moreinfo.cfm?Product_ID=60

THE MAP SHOP
http://www.themapshop.co.uk/europe.htm
(Look for the map that says Euro Mapping – Inland Waterways).

________________________________________________________________________

There are two very popular guide books that describe the Europe
Waterways in amazing details. They are ‘Waterways of Europe’ from
Insight guides and ‘Cruising Guide –Europe’
You can buy them from the following shops.

Waterways of Europe
http://www.amazon.com/exec/obidos/tg/detail/-/0887298257/104-2585597-7196752?v=glance
Cruising Europe
http://www.bluewaterweb.com/cgi-bin/SoftCart.exe/cruisingguides/prodpages/85001.htm?E+mystore4
https://secure.iwashop.com/SHOP/Worldwide%20Waterways.html

Other links that might be of Importance to you.

The Inland Waterways Observatory
http://europa.eu.int/comm/transport/iw/en/site_map_en.htm
Continental Waterways
http://www.continentalwaterways.com/pages/ew-sitemap2003.htm

Useful Keywords

‘Europe physical maps’
‘Europe maps’
‘Europe waterways’
‘Map Europe Waterways’
‘Map Europe Rivers’

I hope this will help you. Please clarify, if you are not satisfied.

Thanks for asking the question and using the Google Answers Service.

Sincerely,
Leader-ga.

Fast, great & enthusiastic answer.  Thanks.