March 2018

Posted by: upperwalnutcreekaustin | March 25, 2018

March 2018

March 24, 2018 – warm and overcast. The last few weeks the weather has swung from normal, with highs in the low 70s, to mid/uppers 80s which breaks records for this time of year.

While walking down to the stream we noticed the inflowing culvert/stream had a good flow and was slightly scummy, though not nearly as much as in the past. The waterfall was flowing about the same rate as last month. Also like last month, the stream flowed past the gravel bar that’s under and just downstream of the bridge at the same rate. It’s possible to cross the stream with dry feet.

The stream is the normal level – all 4 bridge columns are in the water – and all of the excessive algae growths from last month are gone. There is NO surface algae. Why?

While working under the bridge to taking our measurements, I noticed a very faint whiff of dead fish. Hmmmm. After finishing all of the measurements – dissolved oxygen, nitrites, pH, etc – we walked around the sample site and upstream and noticed numerous dead fish, from very small (gambusia – minnows) to palm-sized fish. Very strange. We’ve never seen this before in the 10+ years we’ve been monitoring the site. The actual sample site is about a meter deep, no surprise that there are no dead fish at the actual site. But there were nearly a dozen of dead fish in the shallow water just upstream of the sample site downstream side of the bridge and more under the bridge and even around and upstream of the waterfall, up to near the “sometime island” rocks that are above the stream level. Que pasa? Our measurements didn’t show anything unusual, other than lower dissolved oxygen and cloudy water. Last month with a water temp 3 degrees lower, the dissolved oxygen was slightly more than double its current amount (7.5 vs. 3.5).

Dissolved oxygen – interesting observations
The Feb 2016/2017 dissolved oxygen readings were about the same as last month (Feb 2018). However the March 2016/2017 readings were noticeably higher (6.35 in 2016 and 5.2 in 2017) than this month. Dissolved oxygen is directly related to the water temperature; this month the water temp is 2 degrees higher than a year ago. Summary of historical data for this site on LCRA’s page Is that the only reason for the lower DO?

Pollution can affect dissolved oxygen. This time the oxygen per liter saturation, based on water temperature 19.5 C and DO reading of 3.5, is about 35%. Last month, based on water temp of 16.5 C and 7.5 DO reading, the saturation was about 75%. And in June 2017 with water temp of 26C and DO reading of 4.4, the saturation was 55%. Curious. Warmer water holds less oxygen; after just comparing these 3 readings this month’s DO seems lower than it should be. Again, que pasa? We’re certainly not experts, this could be due to something in nature. Or not.

Apparently whatever happened to cause the fish kill is something outside of what we can measure. Did someone dump something upstream? It hasn’t rained in at least 6 days and the water is not as clear as expected when we checked the transparency (dissolved solids can reduce the sunlight that reaches submerged plants). We’ve never seen it this cloudy. There’s no sediment as would be expected after a heavy rain, the water is just cloudy. The measured transparency is about 0.82 meters; last month it was more than 1.2 meters (the maximum amount we can measure).

3/26 update: saw a post on NextDoor.com (social media for neighborhoods): “…(upstream/north of MoPac) There was some water running from the recent rain, but not much. Today, on the other side of MoPac the creek was roaring with a foul smelling milky water. I’m guessing some manufacturer is flushing stuff into Walnut Creek. Anyone know?”

Someone called 311, here’s the reply:
According to Mr. Span Tindel of Emergency Watershed Protection who is the one who has been working on this issue today. He is stating; “that there was a “Water Main Break”, and what you are seeing is simply tap water mixing with creek limestone which is causing a white sediment to stir. He is saying you may or may not see some dead fish due the chlorine in the tap water mixing with creek water. The white caulk water goes from Mopac to I35, eventually ending in the Colorado River. There is no major cause for alarm!

3/27 Reply from LCRA:

The folks here agree that the limestone sediment stirring in with the tap water would give it a milky appearance. The problem with the tap water is that it has chlorine in it which would kill the fish. The creek should return to normal when the limestone settles out or gets diluted.

As for your site, the low dissolved oxygen could have killed the fish as well. The algae bloom which you reported last month has probably reached the stage where it is decomposing and that would account for the low dissolved oxygen levels. Once the algae bloom goes away, you should see the creek go back to normal as well.

Other observations:

I saw a small amount of surface scum at the sample site briefly, then it was gone. As I walked to the sample site I saw a couple of splashes on the surface, as though as a fish or turtle had disturbed it. Didn’t see the critter as I reached the site. Again, no pigeons or other birds (pigeons sometimes roost under the bridge and their poop is a possible source of e-coli downstream of the bridge). On the downstream side of the bridge, the stream flows through the gravel bar (similar to last month). It’s still possible to walk across the stream by the trail with dry feet, the stream is still that narrow at that point.

The “sometime island” rocks in the middle of the stream that are upstream of the waterfall and (dry) inlet are dry. I didn’t walk all the way up to them though I probably could have done so with dry feet. I was focused more on the dead fish.

E-coli was about the same as last month: with 33 ML samples, the sample site averaged 166 colonies (acceptable for an urban stream) and the control site was almost exactly the same.

Observations:

air temp 23.5 C (74.3 F)
water temp 19.5 C (67.1 F)
dissolved oxygen 3.5 (35% saturation)
pH 7.0
specific conductance
transparency .82 meters
nitrates: 0.3