Lake Erie Annual Tributary Data

The U.S. and Canada agreed to limit the total phosphorus (TP) load to Lake Erie’s central basin, which includes inputs from the St. Clair-Detroit River corridor, to 6,000 metric tons per year (MTA). This was based on modeling of the hypoxic (low oxygen) zone in the lake. The modeling indicated that 6,000 MTA is the maximum load that would result in an acceptable dissolved oxygen concentration of at least 2 milligrams per liter in the bottom waters during the summer stratified period. This means that during the warmest months of the year, oxygen in Lake Erie’s water would be high enough to reduce the “dead zone” that has been cause for concern in recent years. Ten priority watersheds were identified and assigned a target to reduce annual total phosphorus loads to 40 percent of the 2008 load, by water year (October-September). With the exception of the Detroit River connecting channel, these loads are estimated at the mouth of the tributary, using three pieces of information: 1) water quality monitoring data collected at the most downstream stream gage for the river; 2) reported data for point source dischargers beyond this point; an 3) an estimate for the unmonitored area beyond the gage, using a simple ratio for unit-area load. The Detroit River is not monitored directly; instead the load is computed as a sum of upstream tributary loadings and point source dischargers.