You are here

Creekside Story

Our Department gets calls from concerned community members about unusual colors seen in creeks. Some of these vibrant colors are natural, so here’s a quick guide to help identify some of the possible sources that cause these unusual sights.

Our environmental scientists encounter a lot of strange and colorful things in our creeks. Orange slime, purple fluffy goo, and rainbow sheens on the water would all appear to be pollution problems, but often it is just a natural phenomenon. Each of these things can be harmless bacteria just doing what they normally do, which is abnormally fascinating. If you suspect what you are seeing is not naturally occurring and a pollution source might be involved, call the City’s 24-Hour Pollution Hotline (512/974-2550) to initiate an investigation.

Orange Slime

A bright orange slime is often seen where groundwater seeps out of the ground. The slime is called “ferrihydrite” and is basically like rust that is made by bacteria. Iron-reducing bacteria underground turn insoluble iron into dissolved iron hydroxide molecules in the absence of oxygen. These dissolved iron molecules can flow in groundwater to the surface. After the dissolved iron hydroxide molecules flow to the surface, a different group of bacteria use oxygen to oxidize the iron and create an insoluble form of iron called ferrihydrite (the orange slime) as a byproduct. This complex chemical relationship has been present on the earth for hundreds of millions of years, and although it looks gross, it is natural and harmless.

Oil Spill
Oil Spill
Oil Spill

These gooey orange slimes were observed flowing into Little Walnut and Blunn Creek.



This oily-sheen is made of a film of a rod-shaped bacteria called Leptothrix discophora.  The bacteria oxidize dissolved iron and manganese for energy and secrete proteins and carbohydrates.  They are lined up end-to-end within sheathes and are stuck together side-by-side in rows. Sunlight refracts off the sheaths, proteins and carbohydrates to make shiny rainbow colors. The bacteria stick to the surface of water with tiny donut-shaped structures on the sheathes.  The way they are aligned and stick to the surface of the water causes the biofilm to float at the surface and “shatter” like glass when disturbed.  Real oil is greasy and will stick together refusing to shatter like these bacteria. Sometimes these biofilms are present where iron and manganese in groundwater come to the surface, and sometimes they are wrapped around air bubbles in packs of leaves underwater.  These are not bad bacteria, and in fact, they are beneficial because they process metals in water.  They are beautiful, natural and harmless.

Oil Spill
Oil Spill

These oily-sheens floating on water in Spicewood tributary are not oil.

Purple Powdery Fluff

A purple coloration that can develop in stagnant, well lit, waters may be a bacterial group known as “purple sulfur bacteria”. These organisms are capable of photosynthesis, which is why they need sunlight. They don’t use chlorophyll like plants, instead they use “bacteriochlorophyll” pigments as well as carotenoids which make the purple color. Purple sulfur bacteria also require their water to have two key characteristics. The water must have hydrogen sulfide, and cannot have oxygen. They get their hydrogen sulfide from other bacteria that reduce sulfate in the water or soil rich in organic matter and devoid of oxygen. The purple sulfur bacteria are good because they take the hydrogen sulfide (stinky and toxic) and turn it into harmless elemental sulfur. The conditions must be just right for both types of bacteria to thrive. Just like the orange and rainbow bacteria, these organisms are natural and harmless.

Purple sulfur bacteria.
These purple sulfur bacteria were observed at Givens Park.

All of these bacteria thrive under very special conditions and depend on other bacteria for their survival. We usually associate life with oxygen, but these bacterial communities depend on a source of water that is devoid of oxygen. Creekside areas are complex, fascinating and wonderful places full of surprises and interesting biological and chemical phenomenon.

Check out our Water Quality Color Guide to learn more about what’s in our water!

Call our pollution hotline at 512-974-2550 if you suspect a pollution spill.




More Blog Posts

With a healthy creek here and a dirty creek there, how do we measure creek health? The Environmental Integrity Index (EII) is a program designed to continuously monitor and assess the water...

Tagged: eii, environmental integrity index, PAHs, sample collection, creeks, bugs, chemical, macroinvertebrates, scientist

The shoreline of Lady Bird Lake is home to an amazing array of plants and birds, as well as plenty of turtles, and even a few nutrias. Unfortunately, one non-native nuisance plant has set up shop...

Tagged: elephant ears, riparian, lady bird lake, wild taro, invasive plants, invasives

The Mearns Meadow Grow Zone is a beautiful site and would love to be adopted by you!

The Mearns Meadow Grow Zone at Quail Creek Park.

The Mearns Meadow Grow Zone is...

Tagged: mearns meadow, restore rundberg, grow zone

A small-scale lake ecosystem, “microcosm” was designed to learn more about the growth of native cabomba and invasive hydrilla in Lake Austin and Lady Bird Lake.

Have you wondered...

Tagged: cabomba, native plant cabomba. hydrilla, invasive aquatic plant, ecosystem, lady bird lake, Lake Austin, plant cages, reservoir

A recreated stream from Eliza Spring will be home to Barton Springs Salamanders

Long before Zilker Park was home to Barton Springs Pool,...

Tagged: eliza springs, salamanders, salamander restoration, Zilker Park, barton springs, barton springs salamander

Austin’s Grow Zones are beginning to transform into urban forests and aid in carbon sequestration.

Our Grow Zone program began in 2012 in about a dozen parks. The creeks were mowed...

Tagged: forest, riparian restoration, creekside, climate change, aquatic life, Grow Zones, seedlings

About this blog

One of the ways the Watershed Protection Department meets its goals to reduce the impact of flood, erosion and water pollution is through riparian restoration. The community can get involved by adopting a creek, participating in restoration projects, and educating others about the benefits of these areas.

Healthy riparian zones (the land around creeks):

  • control erosion
  • purify water
  • stabilize creek banks
  • regulate water temperature
  • delay floodwaters
  • sequester carbon
  • recharge groundwater
  • provide plant and animal habitat

Creekside Story features creeks that the public can take an active role in supporting restoration efforts.  There are so many creeks to enjoy in Austin.  Check out the Find Your Watershed Viewer to learn which watershed you live in and the Environmental Integrity Index to learn more about their water quality. 



Morris Williams Golf


Shoal Creek


Aquatic bug of the month

Leaf pack


Bartholomew Park

zebra mussels

Tzu Chi Foundation

austin water




tannehill branch

Highland Lakes



grow zone

Stream habitat

red dragonfly

austin creeks

Invasive Species

riparian restoration

watershed protection ordinance

creek restoration

Texas Invasives

creek protection


giant ragweed

seed bank

Adopt –A-Creek

creek score

plant cages

Dittmar park

find your watershed

Lake Austin

Lightning bugs

native plants

Libellula croceipennis

ready-set- plant!

Grow Zones

lady bird lake

Habitat Stewards

invasive plants

neon skimmer


South Boggy Creek



aquatic macroinvertebrates



invasive aquatic plant

Clean Creek Campus


true bugs


clean up

native plant cabomba. hydrilla

Grow Green



standing water






tree seedlings

barton springs salamander

water quality


Austin Water Striders


barton springs

adopt a creek


restoration blunn

Zilker Park

sample collection

Yaupon Holly

Austin Youth River Watch

salamander restoration

Riparian zone


Possumhaw Holly

austin streams

JJ PIckle Elementary


dead wood

environmental integrity index

texas hollies


eliza springs



wetland plants

buttermilk creek


Austin Parks Foundation


Triploid Grass Carp




Ctenopharyngodon idella

shoreline erosion




wild taro

grass carp



Pease Park


austin lakes

monarch butterfly

Lethocerus uhleri



Small Elementary

Barton Creek

volunteer opportunity

elephant ears




maidenhair ferns



walnut creek

ladybird lake

jollyville plateau salamander

Riparian Snag


aquatic life

streambank restoration


stream banks

climate change

seed island

creekside spotlight




restore rundberg

Archilestes grandis

rain gardens

storm water

Keep Austin Beautiful

mearns meadow


Water quality indicators





Clean Sweep

critical environmental features