One hears much talk in riverine ecology about the importance of connectivity. Connectivity between aquatic, riparian, and terrestrial communities promotes heterogeneity, complexity, exchange, and consequently biodiversity and productivity. But, what does that connectivity actually look like?
There are, of course, many manifestations of connectivity. Riparian trees require river flows to scour banks and keep them wet for seedling establishment, and the trees return allochthonous organic material to the river to provide an important base in the aquatic food web. The riparian forests are also corridors through which terrestrial species can travel and seek refuge and food. Another type of connection is between river and floodplain. The importance of a river’s connection to its floodplains has even hit the news in recent years, and not just because humans have an inconvenient habit of building their homes in floodplains only to later have the homes repossessed by the river. In relation to ecology, floodplains have been in the news in their role as valuable rearing habitat for juvenile salmon before the fish face the dangers of the open ocean. Other aquatic and riparian species also rely upon the river and floodplain connectivity for their life history strategies. Perhaps one of the most striking examples of aquatic-terrestrial connectivity lies in marine-derived nutrients that end up in the forests and even wine grapes that occupy upper watersheds of rivers that support salmon runs (Merz and Moyle 2006). Bears and other terrestrial species carry the fish on land, where the nutrients enter the soil and plants of the area. Now, these are what I think of as classic connectivity examples.
When we arrived at the North Fork of the American River, I was quite unsuspecting that I would witness aquatic-terrestrial connectivity first-hand in a way and to a degree I had not imagined. Now, this tributary to the American River is unregulated. Without a dam holding back sediment and modifying the hydrograph, we found the river’s morphology reflective of natural processes. Communities of species were fairly diverse and included foothill yellow-legged frogs, which are indicators of snowmelt fed rivers with natural hydrographs. The river is, however, missing a key species that would have otherwise played an important role in the river’s aquatic food web: the Chinook salmon. With Folsom Dam downstream blocking the passage of salmon into this river, what, I wondered, provided connectivity between the river and terrestrial communities?
As we began our investigations along the river, sampling aquatic insects, searching for frogs, performing pebble counts and discharge measurements, we began noticing a fair number of snakes. These snakes were large and small, dull and brightly colored, swimming and resting on rocks. Now, sure, I’ve thought of snakes (rattlesnakes, primarily) hiding amongst the warm rocks along rivers and thought about what great habitat it is with ample supply of water and rodents. I imagined snakes leaving their forest and grassland homes upslope to grab a quick bite of rodent by the river before returning home. I did not think of snakes as a fundamental link between land and river. Unlike my imagination, the snakes we were seeing were garter snakes and seemed to be quite in their element. What were they doing, swimming around? Only when I followed shouts of excitement did I see that, in fact, they were swimming around because they were fishing! Observing a sculpin halfway down the gullet of a snake, it dawned on me that, given how many of these snakes we were seeing, their role in the riverine food web was not likely negligible. This experience brought home that connections between land and water could be anywhere and may exist where you least expect it – I’ll be keeping my eyes and mind open to making new connections like this in the field.
Signs of a natural flow regime on the Clavey River
Few rivers of the Sierra Nevada are undammed. Of these, only a fraction is relatively unmodified by other human interactions. One such system is the Clavey River, the largest tributary of the Tuolumne River. Within a watershed of 231 km2, its 50.5 km channel flows north-south and then makes a swift turn to the west at Jawbone Ridge before entering the Tuolumne. Without a dam, water arriving as rain and snow passes through the Clavey system to the Tuolumne unchecked. Large rain events produce dramatic spikes in the river’s flow and the later spring melting of snow results in gradually receding flows into the summer. Both signatures can be seen in its mean daily flow hydrograph (Figure 1). Due to its natural flow regime and relatively little other human intervention, the Clavey is highly productive and biologically rich, consisting of intact native biotic communities (Moyle and Randall 1998).
Floods
The relief of the watershed produces a high-gradient river with high-energy flows capable of transporting large materials downstream, such as the large logs in the flood debris. From year to year, floods shift sediment around to remove willows and expand a bar here, or move a boulder there, making the dynamic morphology that is characteristic of rivers with natural hydrographs. Large cobble bars occupy the inside of meander bends, potholes scoured by entrapped sediments dot the bedrock channel, and knickpoints marked by waterfalls have retreated upstream over time through the erosive power of sediment and water. Also characteristic is the regular disturbance of the riparian vegetation by floods. Such signatures of a natural flood regime are less prevalent on the Tuolumne given the flood regulation provided by O’Shaughnessy, Cherry Valley and Lake Eleanor dams upstream, as well as natural differences in geology and stream gradient.After observing the Tuolumne River, a hike up the Clavey River can reveal substantial differences between the regulated (Tuolumne) and unregulated (Clavey) rivers. For one, evidence of floods is quite apparent on the Clavey. The highest flows of the previous season are revealed in the wrack line of woody debris left as the flood waters receded. This line sits high above the summer water line, in amongst the boulders at the edge of the channel. An even higher line marks the large 1997 flood, which scoured grass and soil from the steep banks lining the river channel.
Heterogeneous channel morphology
Adding diversity to the typical riffle-pool sequence of mountain streams, the natural flow regime supports diverse habitat features. One such feature is an extension of a pool lying alongside the main flow of the Clavey at the first bend upstream from its confluence with the Tuolumne. These side pool habitats generate heterogeneity in the system, providing unique habitats for native fish, amphibians, and aquatic insects. They contrast strikingly to the swift moving boulder and bedrock dominated main channel. Riparian vegetation establishes in the sediments deposited by the slower moving water. Willows as well as sedges, button bush, and other wetland species create lush habitat that provide both food and protection to aquatic organisms. The effect of flood disturbance that maintains and fills these side channel and pool features is combined with groundwater inputs that sustain the habitats through the summer months.
Tadpoles of the native foothill yellow-legged frog can be found along with juvenile pikeminnow, California roach, hardhead, and Sacramento sucker. Simply the presence of the foothill yellow-legged frog in this system indicates its natural hydrograph, as this specialist species depends upon an annual snowmelt signature in the river’s flows that is unaltered by dam regulation. Though the primary habitat of the foothill yellow-legged frog lies within the riffles, boulders and cobbles in the main channel, the pools of this side system provide additional beneficial habitat. Their warm waters – circulating much slower than the fast-moving water of the main channel – promote primary production including algae growth, which juvenile fish, tadpoles, and insects feed upon. Learning to read the signs of a natural flow regime and link them to the biologic communities they support informs the work of physical and biological scientists alike, and the Clavey is, I found, a great place to do that.
References
Merz, J. E., and Moyle, P. B. (2006). "Salmon, wildlife, and wine: marine-derived nutrients in human-dominated ecosystems of central California." Ecological applications, 16(3), 999-1009.
Moyle, P. B., and Randall, P. J. (1998). "Evaluating the biotic integrity of watersheds in the Sierra Nevada, California." Conservation Biology, 12(6), 1318-1326.