Can someone please direct me to a tutorial where I can learn more about how to model flooding using data from gauging stations along rivers using QGIS?
If you use GRASS GIS as backend in QGIS (via QGIS-GRASS toolbox or Sextante plugin), you can use a range of hydrological tools. See http://grass.osgeo.org/wiki/Hydrological_Sciences for options.
The GRASS recommendation from @markusN is a good one. Another option, although it's not integrated into QGIS, is the Gerris Flow Solver. GFS is a tremendously powerful hydraulic and hydrological modeling tool. From the site:
There is also a very detailed tutorial on the Karamea river, in New Zealand. If you follow that example you will learn a lot about the software, flood visualization, and hydraulic modeling.
Have a look at Crayfish for QGIS
"Crayfish is a plugin (extension) developed by Lutra Consulting for the free and open source GIS platform Quantum GIS (QGIS).
The Crayfish plugin aspires to be a complete set of pre and post-processing tools for hydraulic modellers using TUFLOW, ISIS 2D and other modelling packages."
Personally, I have no experience with it.
Sort of depends where you are (which country) on what software and methods are usually used or are standardised on - some councils, state governments and governments have required methods. Also what sort of area are you looking at modelling ? Urban, rural ???
eg for Australia have a read of the www.arr.org.au site. They are in the process of releasing updated guidelines and some state governments are in the process of releasing their required methods. drafts chapters of their new releases www.arr.org.au/downloads-and-software/chapters/
QGIS and SAGA are very useful
WBN have released a tuflow plugin for QGIS back in april (2014)
They also have a tutorial for using qgis to prepare models for tuflow http://www.tuflow.com/GIS%20Platforms.aspx?QGIS_and_SAGA http://wiki.tuflow.com/index.php?title=Main_Page
Drains for urban pipe and drain flow (required by some councils) also for flows into pipe and drains for tuflow www.watercom.com.au/download.html
Can download hecas for free
In the US be very careful about units as some states use metric and some US imperial for their data.