Geology: Hydrographs
Have you ever looked over a rail on a bridge to peek at the height of a river? If you have, you may have noticed that the river level is low after a drought and high after a long rainstorm. The team from USGS installed a stream gauge along the White River in 100 Acres to measure its height, allowing us to observe changes without having to be in the Park.
On the right, you can see the USGS hydrograph which shows the measurements taken by the stream gauge (pictured on the left) over a period of 60 days. The peaks tend to occur after big rainstorms or after rising temperatures cause snow to melt. Some of the biggest floods happen as snow melts throughout the watershed in late winter - add rain and watch out! It's important to note that the peak in stream height often comes significantly later than the peak in precipitation or snowmelt. This is because the water usually travels a long distance through the watershed, which is mostly agricultural.
Also notice that between the peaks the level drops as low as four feet. At this point the river is at about the same level as the water table, and the river is at what is known as base flow. The water represented by the region that we've highlighted in blue comes mostly from groundwater, while the additional water is runoff from surfaces like farms, roads and parking lots.
The chart above is the hydrograph displayed on the IMA Dashboard. The yellow and red regions within the chart indicate the action and flood stages as determined by the National Weather Service. Local weather service stations generate alerts to the public when river levels reach these stages at points that they monitor, based on their knowledge of how river height relates to hazardous road conditions in an area.
Check back soon to read more about flooding in 100 Acres.
