Interesting question

Unfortunately for Slovenia only...

Tables for the current water level, discharge, and temperature:

1 day backwards from the current date (10 minutes resolution):

http://www.arso.gov.si/vode/podatki/amp/Ht_1.html

7 days backwards from the current date (30 minutes resolution):

http://www.arso.gov.si/vode/podatki/amp/Ht_7.html

30 days backwards from the current date (30 minutes resolution):

http://www.arso.gov.si/vode/podatki/amp/Ht_30.html

Tables for historical data

Daily time series with monthly extreme values extracted for the water level, discharge, temperature, and suspended material:

http://vode.arso.gov.si/hidarhiv/pov_arhiv_tab.php

Monthly time series with monthly extreme values extracted for the discharge and temperature:

http://www.arso.gov.si/vode/podatki/arhiv/hidroloski_arhiv.html

Except for the last link It's written mailnly in Slovenian but i can help with translation if/when needed.

The United States Army Corps of Engineers maintains many gauge stations across the USA. You can find river stage and discharge records from these stations at the following website:

http://rivergages.mvr.usace.army.mil/WaterControl/new/layout.cfm  

Real time water quality data for nitrate, DO, SC, temperature, turbidity, discharge available for about 70 Iowa stream sites here: http://iwqis.iowawis.org/