If you build it, will they come? That is the question being asked by the Corps and other organizations that have come together to form the Middle Rio Grande Endangered Species Collaborative Program. If suitable habitat is built in the Middle Rio Grande, and river flows are adjusted to more closely mimic natural flows, will the endangered Rio Grande silvery minnow become abundant once again?
The silvery minnow is a small, three-inch long fish with a tiny mouth and eyes. In the early 20th century, the minnow was common along the Rio Grande, from above its confluence with the Chama River almost to the Gulf of Mexico. The river throughout much of this stretch was a wide, braided stream with shifting sand bars and a wide, regularly inundated floodplain.
However, dam construction, jetty jack installation, channel straightening and other projects built during the last century to reduce flooding have altered the river’s flow. Where once peak spring flows overtopped the river banks and flooded the valley, now the flows are confined to a narrow, rapidly eroding channel. The result has been the development of a single-channel river with relatively swift current, a dry floodplain that is too high above the river to be inundated by spring floods and a river with few back channels and eddies creating suitable minnow habitat. Predictably, the minnow’s range steadily contracted. By 1994, the minnow was present in only 5 percent of its former range and it was officially listed as a federally endangered species.
Biologists were unsure what could be done to save the minnow from extinction. Not enough was known about the biology of this fish to discern which changes to the river had been most damaging. One idea that river managers had was that damming and channelization had eliminated shallow channel and overbank habitats essential to minnow reproduction.
To test this idea, the Corps joined with other members of the Collaborative Program, including the New Mexico Interstate Stream Commission (ISC), the U.S. Fish and Wildlife Service, the Albuquerque-Bernalillo County Water Utility Authority, the U.S. Bureau of Reclamation, the New Mexico Department of Game and Fish and the Middle Rio Grande Conservancy District. Together, they organized the construction of a series of high and low flow channels cutting across islands and river banks to help restore a dynamic, complex floodway in the Middle Rio Grande.
Such a floodway has a diversity of channel habitats and flows suitable for silvery minnow reproductive and early life stage needs.
In collaboration with water users, the Corps has also experimented with artificially lengthening spring high flows and re-introducing native floodplain vegetation. All of these actions should, in theory, have dramatically improved minnow habitat and led to increases in minnow populations. But did they?
In spring 2010, the Corps and other members of the Collaborative Program wanted to find out. But unforeseen and unavoidable obstacles to the contracting process meant the contractor could not be hired in time to conduct low-intensity monitoring during the critical spring runoff season. So, the co-chairs of the Collaborative Program Monitoring Plan Team, Ondrea Hummel (Corps) and Anders Lundahl (ISC), rapidly fielded a multi-agency team of biologists, hydrologists and geomorphologists to assess whether minnows are living and breeding in their recreated habitat, and whether the target stream flow levels have been achieved in the different kinds of restored channel environments.
What they learned is encouraging: the habitats designed with minnows in mind were exactly what the minnows liked. More than 1,600 minnows and countless minnow eggs were identified at the 20 restoration sites monitored in 2010. Flows in various channel features were within their target ranges, and native vegetation was becoming established to the exclusion of non-native species.
According to Corps biologist, Michael Porter, who is an expert in silvery minnow biology, “the habitat that was restored turns out to be just what the minnows need for spawning and rearing their young.”
The low-intensity monitoring effort was so successful that the Collaborative Program has decided to keep it “in house” in the future.
According to biologist Hummel, “performing the monitoring ourselves allows us to be aware of how the sites are maturing each year and how they are being used. This knowledge informs future habitat restoration efforts and techniques.”