From: Gregory McIsaac
As per our discussion yesterday, I am attaching a collection of slides that demonstrate changes at river sites. Feel free to use any of these, suggest changes, or if you would like to reformat them, I can send you the data.
I tend to emphasize the importance of water flow to loads, but it is obviously not the only factor.
The decline in TP loads in the Chicago area and the increase on the Sangamon are point source related.
The cause for the increased nitrate in the Illinois section of the Rock is still uncertain, but increased rainfall, increased irrigation and groundwater flow paths are probably relevant factors.
The shift in the relationship between water yield and nitrate yield on the Mackinaw may be a sign of conservation efforts having a favorable impact, but I am not certain if that is the cause.
I think it is important for people to understand the difference between monitored drainage areas and HUCs. I included some slides along those lines in case you wanted to make that point in the presentation. There can be considerable uncertainty introduced when extrapolating from the river monitoring data to the HUC areas. I unsuccessfully tried to convince IEPA to abandon the HUCs. HUCS were a great idea before we had GIS. Now that we have GIS, HUCs seem like an unnecessary and unhelpful approximation. If we are trying to understand what is happening in the rivers, I think it best to use the actual river monitoring locations and their drainage areas, not the HUCs.
Best regards,
Greg McIsaac