US Scientists Establish that Stream Biodiversity Declines at Extremely Low Levels of Urban Development



Ecological “Tipping Point” is Dramatically Lower than Previously Believed

According to an article published by Science Daily, a new study from biology researchers at Baylor University (in Texas) and the University of Maryland-Baltimore has found that there are consistent and widespread declines in stream biodiversity at lower levels of urban development more damaging than what was previously believed.

The study found that aquatic life actually shows significant loss of biodiversity with less than two percent of developed land in a watershed.

Approximately 80 percent of the biodiversity loss came between 1/2 and two percent of impervious cover, and the remaining 20 percent of loss came between two and 25 percent.


To Learn More:

To read the complete article in Science Daily, click on Widespread Stream Biodiversity Declines at Low Levels of Urban Development

Journal Reference: Baylor University (2011, June 13). Widespread stream biodiversity declines at low levels of urban development. ScienceDaily. Retrieved July 2, 2011, from


Impact of Increasing Urbanization on Stream Corridor Ecology

The image below is reproduced from Stormwater Planning: A Guidebook for British Columbia. Based on Washington States research by Richard Horner and Chris May in the mid-1990s, it illustrates the cumulative effects of increasing impervious area in a watershed combined with loss of riparian corridor integrity (as shown in the first two rows), alter the natural Water Balance and impact stream corridor ecology (as shown in the last two rows). To download a PDF copy, click on the image.

This early research established that ~10% impervious cover is a threshold level at which fisheries biodiversity and abundance are initially and significantly impacted. By ~30% impervious cover, most urban watersheds in the Pacific Northwest may be unable to sustain abundant self-supporting populations of cold-water fish.



A Washington State Perspective

“Such studies are rare indeed and should receive broad circulation.  This study adds to the work that Jim Karr, Derek Booth, and Chris May have done to define stream health with Tom holz (140p)increments of watershed disturbance,” observes Tom Holz. A recognized stormwater authority, Tom Holz is well-known in Washington State for his tireless efforts in leading change in the field of rainwater management and green infrastructure.

“The implications of this study regarding standards for development are truly sobering.  Our touch must be as light as those builders who used to prepare land with hand tools.”

In the late 1990s, Tom Holz coined the acronym ZID – that is, Zero Impact Designs – to describe an approach that sharply reduce the “effective impervious area” of new development with practices such as eco-roofs, roof gardens, rain barrels, alternative paving surfaces, soil amendments, bioretention, reforestation, and filter-swale systems.


Why Department of Ecology Drainage Standards will not Protect Puget Sound 

In March 2011, the Thurston County Board of County Commissioners requested a seminar by Tom Holz on “Why DOE drainage standards will not protect Puget Sound”.  The seminar described how low impact development is the only path to protect the Sound. 

According to Tom Holz, “The Department of Ecology apears to be on a path to continue using the same standard for development for the next five to eight years that has been used for the last decade.”

“DOE calls it the ‘flow-duration’ standard.  It more accurately should be described as the 0/100/100 standard.  That is, DOE will require ‘0%’ forest set-aside, will allow ‘100%’ hardened surfaces, and will allow ‘100%’ runoff of precipitation falling on a site.” 

“As almost everyone knows, healthy streams are found in watersheds that are 100% forested.  Stream channels begin to destabilize following the clearing of about one-third of its watershed.  Thus DOE will allow development that will result in exactly the opposite of a healthy watershed,” concludes Tom Holz.


To Learn More:

The seminar is posted on YouTube. To view Tom Holz, click on the two links below:

The first link is about 52 minutes (fast forward to the 4.13 minute mark to get past the set up). The second link is closing and discussion with decision makers and public. “It’s a bit dry so make a bowl of popcorn,” recommends Tom Holz.


The View from British Columbia

Pamela zevit (180p)“For those interested in a comprehensive look at integrated stormwater planning challenges and issues from south of the border, take some time to watch the video of the seminar that Tom Holz did for Thurston County,” states Pamela Zevit, formerly with the BC Ministry of Environment and now an independent consultant.

“Tom deserves the title Low Impact Man.  While there is certainly room for debate about what Tom is and has been proposing for decades, the seminar provides a good snapshot of the issues and potential solutions. Also some insight into the dilemmas and regulatory hurdles being experienced in Washington State which we share a great deal of ecoregional issues with.”

“For those involved in land use planning in British Columbia, the political challenges will sound familiar. See if you can catch the differences between US and Canadian terminology in land use planning,” concludes Pamela Zevit.


Posted June 2011