It’s not often that you get to see mallards swimming amongst the trees in the bosque, or find fish swimming in the litterfall tubs, or wade through waist deep water to get to a precipitation gauge, or raft over pitfall traps, but these were exactly the sort of experiences we had this spring when the high river flows led to flooding in the bosque. It was a fantastic opportunity for us to experience historical conditions and to learn more about how this ecosystem is changing.
BEMP monitors many different aspects of the bosque ecosystem. One thing that we don’t regularly get to monitor, but it’s one of the critical aspects that we need to better track, is mapping how much of the bosque floods when the river flow is high. Another critical aspect to investigate is: of those areas that do flood, how many flood in a way that supports resiliency in the ecosystem?
What does this mean?
It means that in the past, the force that created, destroyed, recreated, and supported the bosque was the overbank flooding. When snow melted in the mountains in the spring, large amounts of melt water would rush down the Rio Grande and there would be too much water to stay in the active channel of the river. This “extra” water would flood into the areas alongside the river, not only supporting existing plants, but allowing new plants (like cottonwoods and willows) to germinate. The floods were the driving force behind young forests, grassy meadows, wetlands, willow swales, and more. The flood waters also provided habitat for animals, by creating backwaters, or pools of slowly moving water. As critters took advantage of these calmer waters, the water would also seep into the ground, creating a storage bank of water for the grassy meadows, forests, and wetlands to tap into after the floods had retreated. All of this meant that the ecosystem was resilient, or could “bounce back” following different disturbances. If a fire swept through a part of the bosque, flooding the following year would provide the resources needed for the ecosystem to recover.
Now that we have tamed the river, we no longer have the extensive overbank flooding that is so essential to many of our native species. Occasionally, we do have years when the river has enough water to flood up over the banks and into the bosque. When that happens, is there enough water to provide habitat for endangered species like the Rio Grande silvery minnow? Is there enough power in the flows to scour the ground and provide clear substrate (or soil) for cottonwoods and willows to germinate? Or is it just enough water to lay down a gentle coating of mud on top of the plants that were already growing? In today’s altered system, these are some of the things we need to know in order to figure out what the future holds for our riverside forest.
To get at these questions, BEMP started doing “inundation investigations” following the high river flows in the spring of 2017. “Inundation” means flooding, so this was a starting point to map out which BEMP sites flood at high flows, and how much of each site floods. BEMP staff took pictures of the flooding at sites in April and May, and some sites were mapped to show how much of the site was under water. Rio Grande flows reached 5630 cubic feet per second (cfs) in May 2017, and there was flooding at 16 of the 32 BEMP sites, which was a new record! Some of the sites were completely flooded, while others only had flooding in spots. Some areas had flood waters from the river’s edge all the way to the levee, so that the forest became a lake-like area for mallards, turtles, and fish. Pictures and maps of the flooding will be used to determine the extent of silvery minnow habitat that was available. Post-flood monitoring will allow us to determine the impacts of the flood on vegetation and arthropods living in the bosque. At this point, we are finding that most sites did not have strong, scouring floods, which means there are few to no new cottonwoods or willows coming up at BEMP sites, but we’re excited to see if the floods did have a positive impact on the existing plants and critters of the bosque.
Blog Post by Dr. Kim Eichhorst, BEMP’s Science Co-Director. Image at the top is early May flooding at the Bosque Farms BEMP site.