OnionCreek travels
75 miles from its headwaters near Blanco, Texas to where it meets
the Colorado River east of Austin Bergstrom International Airport. It is a unique watershed that crosses two eco-regions,
the Edwards Plateau and the Blackland Prairies, and contributes
a substantial amount of the total recharge to the Edwards Aquifer.
The upper watershed, in the Edwards Plateau, is relatively pristine
and dominated by large ranches that are increasingly being developed
as subdivisions. Crossing IH35 in south Austin, the creek flows
through McKinney Falls State Park and into the Blackland Prairie
eco-region, where the watershed becomes more agricultural and
densely residential. These lower portions of the watershed are
currently coming under increasing development pressure associated
with ABIA and Highway I-130.
The City began monitoring
water quality on Onion Creek in response to contact recreation
concerns in McKinney Falls State Park, where high levels of fecal
coliform bacteria (in part from the Williamson Creek Wastewater
Treatment Plant immediately upstream) resulted in a swimming ban
beginning in 1981. The monitoring program was designed to evaluate
the re-opening of the park to swimming in 1993 after the plant
was closed. Study results indicated that swimmer safety was maintained
during baseflow, but can be impacted by heavy rains.
Since then, the study has
been expanded to provide a more comprehensive look at the watershed,
with monitoring sites in the upper reaches near Blanco and Driftwood,
and continuing downstream to the confluence with the Colorado
River. Water quality parameters such as nutrients and bacteria
are measured three times per year, while habitat, benthic macroinvertebrates
and diatoms are assessed twice, in June and August of each year.
