How to read website data by auto-filling required fields from C#

I want to read water level data from the following website for my research purposes: http://ift.tt/1vMAzVI

When I fill the required fields of this page, it redirects to a new page: http://ift.tt/1LZ4n5r

I need the data regularly and for all stations. I have some experience in C# along with HTMLAgilityPack, but not get any point How can I do that using C#. Can anyone help me in this regard?