The U.S. Geological Survey’s WaterWatch displays data relating to hydrological events throughout the country in real time.  Interactive graphs, maps, and tables show streamflow, flood areas and drought areas. Maps and data can be viewed on both national and state scales.  Past streamflow data can be easily accessed. Annual summaries of streamflow data are also kept on this website. All this data can be displayed in a surprising number of ways.  WaterWatch’s toolkit section features 21 different graphs, charts, tables, and maps displaying and comparing streamflow data. My personal favorite is the customizable Streamflow Map Animation that shows streamflow data on a map over a period of time:

https://waterwatch.usgs.gov/index.php?id=ww_animation

The USGS stores enough water data to drown any student or professional researcher.  Such data is collected by over 3,000 USGS stream gauges throughout the country. Streamflow data is important to measure on such large scales because rivers and streams connect surface runoff to to large water bodies and groundwater.  Streamflow for all streams is therefore vital to the water cycle.