Flood Plains of the Lower Schuylkill Watershed

Using ArcGIS and US Soil Survey's online mapping tools, I was able to obtain some visuals depicting the Lower Schuylkill River's flood plains and high risk flooding areas. The filter I used on ArcGIS is called "USA Flood Hazard Areas." It was created by FEMA and displays the data from their Flood Insurance Rate Map. The flood plains are primarily the land immediately next to the Schuylkill River and some but not all of its tributaries. As can be seen in the legend on the left, the light purple areas represent a 0.2% chance of annual flooding, while the darker purple areas represent a 5x higher rate at a 1% chance. 

One curious aspect, that could simply be an error of some sort as a result of a technical issue, data entry issue, or due to different counties' data conflicting, is the very large chunk of light purple area seen just to the right of the center of the map above. This area measures out to be 37 square kilometers or 14.3 square miles, covering multiple entire towns including Narberth, Penn Valley, and Bala Cynwood. There are some tributaries that run through this area, but I don't think they are enough to cause the entirety of such a large area to be at risk of flooding. Furthermore, I don't think flood plains could have such straight lines that coincide exactly with the county borders for Montgomery county. It also seems to be a clear outlier in terms of shape when further zooming out and comparing to other flood plains across the country and in the greater Lower Schuylkill Watershed, as can be seen in the above image. With this in mind it is a safe assumption to disregard that section of data. That is not to say there is no flood plain within that area, as near the tributaries within the area there most likely is, just not the entirety of the highlighted zone. Considering that this ArcGIS map layer spans across every acre of the USA it's not unrealistic that an error could exist.

The tributary I have been focusing on (seen above in the center of the image) that runs through Saunders Woods Preserve, according to this data does not have land classified as flood plains surrounding it. To verify this, I examined US Soil Survey's online soil map. The soils of flood plains, also called alluvial soils, are typically made up of high proportions of small particle matter such as silt, sand, and clays, eroded down and deposited by the flood waters. There are many different soil types listed in the area, however most of the ones in immediate proximity to the tributary are classified as a type of loam, which is composed of a relatively equal mixture of particle sizes including the larger sized gravel. This likely confirms the data from ArcGIS that the Saunders Tributary's surrounding banks and land does not experience flooding often. From firsthand experience I know that it is a relatively small and slow flowing tributary with steep banks in some areas, so this information does not come as a total surprise.