Distance Decay Tool

[SSmi]

I am working on a project where the hypothesis is along the lines of: the further away the sampling point is from the nearest river, the better the water quality will be. I have plotted the water quality points and coloured them appropriately, but now want to find out if there is a correlation between where the river meets the sea and the water quality.

It is easy enough to calculate distance from the sampling point to the mouth of the river, but how can I see if there is a correlation for each point to its corresponding river?

Additional Info:

The rivers and coast are line shapefiles, so for easier analysis I made point shapefiles where the coast and river meet (to indicate where the mouth of the river is).

Thanks for the help

[ChadKopplin]

I would add Land Use, Land Cover, Surficial Geology, and Bedrock Geology,  and Slope into your model as well, because these will be major players in the water quality of any given area.

[DanPatterson_Retired]

Chad has covered some of the surficial components, but I will add how was water quality determined? was the assessment based on bacteria or turbidity? was it groundwater quality and if so, at what depth?  proximity to population? etc.  Correlation would be  somewhat further down the list of concerns.

[SSmi]

Thanks Chad and Dan, appreciate the help. The data are actually only measuring two species of dangerous bacteria that come from sewage - it's hardly everything to do with water quality and it's not meant to be. But these factors are issues to do with the research design (which will become much more complex and revised). Right now I'm after the tools that will allow me to interrogate the data as it stands.

[DanPatterson_Retired]

If you have a DEM, I would first perform a watershed analysis to delineate which points fall within which watersheds.  I am not clear on the association of the sample points to the rivers and if they were default locations or chosen for a reason.  Tools do exist particularly in the spatial analyst extension to model distance based operations (ie cost distance, path distance, euclidean distance etc)

[ChadKopplin]

You can use the Near Analysis tool in ArcToolbox>Analysis>Proximity>Near to calculate the distance from the points to the river line, then see if there is a correlation between the distances and the water quality values.  However, this tool requires and Advanced license.