ephemeral

Intermittent Rivers - Kim Fike

Systems with Periodic Flow

//**Introduction**//  Intermittent streams and rivers are found on every continent and are characterized by their repeated onset and cessation of flow. They are not restricted to arid regions alone, they occur in most ecosystem types worldwide (Larned //et al//., 2010). The total length and discharge of the global river network is comprised of more than 30% intermittent streams (Tooth, 2000) For example, roughly 70% of the river channels in Australia (Sheldon //et al//. 2010), 25-40% of total river length in France (Snelder //et al//. 2013), and more than half of the total river length in the US, Greece, and South Africa have temporary flows (Larned //et al.// 2010). These temporarily flowing systems provide essential ecosystem benefits, crucial habitat, and are an important role in the cycling of nutrients within arid and semi-arid regions.

 As a result of climate change, the number and length of intermittent rivers (IRs) are expected to increase in regions that experience drying trends (Palmer //et al//., 2008). Anthropogenic impacts including land use change, water abstraction, and stream impoundments are also artificially increasing the number of intermittent streams worldwide, leading to severe ecological and economic consequences (Meybeck, 2003). The majority of the world's rivers are altered in some form, and many of them are undergoing artificial drying events that were not a part of their historical natural flow regime.

 Community ecology, biogeochemistry, hydrology, and river management practices have been studied extensively for decades within perennially flowing streams. However, there has been significantly less attention and fewer studies on intermittent systems. Intermittent systems operate completely different from continuously lotic ecosystems. Previously, the networks were regarded as relatively insignificant aspects of the overall ecology of an area. However, recent attention on temporary streams have shown that they have substantial economic value and ecological importance. Since this is a relatively //new// discipline within the field of freshwater ecology, there is not very much legislative support providing protection for these periodic systems. As a result, these they are experiencing high levels of degradation due both to human induced and natural processes. Management practices for intermittent streams cannot necessarily be adapted from perennial systems because of the number of dissimilarities. They are in a category of their own, and it is in need of improvement.

Left image of an intermittent river in Ft. Huachuca, AZ. Image can be found at: []  Image on right is from the USGS and can be found at: []

In termittent rivers persist in a wide variety of physical states depending on geographical and climatic conditions. Since they are found on every continent, their channels carve through a wide variety of ecosystems. Rainforests, deserts, arid-land and temperate grasslands, deciduous and evergreen forests, tundras and more, all experience streams with periodic flow throughout some part of the year.
 * // Physical characteristics // **

Arid to semi-arid regions exhibit extensive networks of arroyos with lengths that vary from 5-200 km long and have cross sections that are characterized by relatively flat riverbeds and steep nearly vertical banks often lacking in vegetative cover (Bull, 1997). Arroyos remain continuously entrenched and have highly consistent alluvium that is deposited over the sometimes century-long life of these systems. Gullies, in contrast, are relatively short systems due to their quick periods of formation and dissolution. Episodic streams that flow through heavily vegetated stretches often have streambeds with large deposits of organic matter that have accumulated throughout the dry periods. Higher elevation systems will often have variable streambed cross sections with rocks and soil of varying nutrient composition throughout the length of the stream.

 Intermittent rivers share some similar characteristics of perennial rivers such as the hyporheic zone, which is a region beneath the streambed and along the sides where the groundwater and surface water can mix during lotic periods. The interactions between the groundwater and surface water are important for organisms that live in the hyporheic zone as well as for nutrient cycling processes. These zones are often seen as refuges for organisms such as frogs, toads, salamanders, and other burrowing animals after a retreating period.

 Figure 1 displayed to the right shows the expansion-contraction cycle of the Thouaret River in France. In this system, which behaves like many intermittent systems, some sections of the river maintain continuous flow (blue), some dry up entirely (red), and other stretches contain isolated pools of varying sizes and depths (yellow). This variability over time creates numerous habitat types that are capable of supporting of a wide variety of aquatic, semi-aquatic, and terrestrial fauna throughout different times of the year.

 The variability of flow is one of the most important factors determining ecological patterns within rivers (Power //et al//., 1995). The repeated cycles of flowing and drying periods are an extreme form of river variation that has significant effects ecologically. The intermittence creates fluctuating longitudinal dynamics with advancing and retreating wetted fronts, hydrologic connection and disconnection of remaining pools, and flood pulses of varying intensity and perpetuance of flow.
 * // Hydrology // **

Figure 1: Phases of intermittency of the Thouaret River in France, Datry //et. al// (2014)  <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Advancing fronts can occur with various levels of intensity, ranging from slow, steady increases in water level, to sudden flash flood pulses that occur in a number of arid-land environments. Aquatic habitat contraction often leaves behind aquatic patches ranging in size from small scour pools to long reaches with high permanence of water. Terrestrial patches can range from elongated reaches with zero flow to exposed boulder tops within inundated environments (Datry //et al//. 2010). The shifting of patches from dry to wet vary in frequency from less than a day to more than a year.

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">The lotic periods within one stream change on month to month basis, but they are also incredibly inconsistent from year to year. The frequency of riverbed inundation, timing of when the flows return and recede, and length of flow can be drastically different on an annual basis. Figure 2 to the left, shows a hydrograph of the Rio Puerco, an intermittent river in central New Mexico that is a major tributary to the Rio Grande. The graph displays the average daily flow from 2005-2010. Even with a short period of record, it can be seen that flows occur at different times of the year, but not necessarily every year at that time. <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Figure 2. Hydrograph for the Pecos River, NM from 2005-2010

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">The graph demonstrates an obvious trend of increased flows during the summer months, primarily as a result of the monsoon season. However, the wide range in flows and duration of flow is different for each year. In 2006, (the orange line) flow briefly exceeded 5,000 cubic feet per second (cfs) and exceeded 1,000 cfs on four separate occasions throughout the year. The highest flow from any other year during this period of record was in 2010, which was around 900 cfs for a matter of a few days. Years with multiple high flow events that persist in channel for longer durations offer different habitat benefits than the same river channel with no or low flows throughout the majority of the year. If the above graph displayed 50 or more years, one could imagine the spatiotemporal variety of flow this intermittent river would display.

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">High-flow pulses like the 2006 flash flood shown in Fig. 2 can cause significant ecological changes such as massive bank scouring, vegetation removal, and excessive sediment transport. Sediment plays a large role within intermittent rivers. There is always a disequilibrium between periods of stream in-cutting and stream aggradation (Bull, 1997). Erosion is primarily in the form of headcut incision and recession (Patton and Schumm, 1980). It is common for much of the sediment that is removed at the headcutting to be deposited downstream where the gradient lowers and the channel widens leading to aggradation. Arroyo cutting and filling is one of the means by which sediment is transported out of semiarid basins with naturally high sediment yields. In a series of case studies examining the patterns of aggradation/degradation in arroyos in the southwest, researchers discovered that there were erosional reaches directly below sections that had aggraded nearly to the valley floor (Patton and Schumm, 1980). The channels within the erosional reaches eventually widened to the point that then began to trap sediment from upstream erosional events. The greater the distance from the nickpoint, the greater the sediment concentration and the greater the rate of deposition.
 * //<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Geomorphology //**

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">This process of cutting and filling is a fundamental process that is influenced by changes in vegetation or climatic fluctuations. Intermittent rivers in areas where heavy grazing practices exist often experience more erosion than in under pre-grazing conditions. As a result, the reaches with low width:depth ratio often extend further, transport more sediment, and degrade at a faster rate than they would have under natural circumstances. This same process of excessive degradation occurs in areas that are experiencing more variable monsoonal activity. Widely spaced thunderstorm events with short bursts of high intensity rainfall can cause extensive streambed degradation. The long duration between storms allows soil moisture to decrease, loosening up sediment particles and making them vulnerable to the next precipitation event.

//**<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Biogeochemistry **//

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">In the absence of water, biogeochemical processes such as decomposition, mineralization, and ingestion of deposited organic matter are slow. Riverbed moisture content in the absence of flow or sustained precipitation events is typically <5% (Larned //et al.//, 2007). Flow pulses act as a catalyst for the re-activation of microbes, re-hydration of organic matter, and the dissolving of nutrients (McIntyre //et al//. 2009). Several studies on aridland soils are being pursued in order to identify relationships between the size of the pulse and the ecological, biogeochemical, and physiological responses (Austin //et al.//, 2004). Short pulses generate responses mainly limited to surface soil microbial activity, and longer pulses tend to catalyze additional responses that occur deeper within the hyporheic zone. Intermittent rivers respond similarly to aridland soils, with their activity triggered as a result of the reappearance of moisture. Total saturation of the riverbed, just like aridland soils, inhibits aerobic microbes therefore slowing down some of the mineralization pathways. This transition from anaerobic to aerobic microbes can vary from seconds to weeks (Schwinning and Sala, 2004).

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">A succession of events within intermittent river biogeochemistry is initiated by rainfall, upwelling groundwater, or inundation by downstream flow. The initial wetted front moves nutrients, debris, and minerals downstream into areas where debris tends to accumulate such as the tops of sandbars or pooling areas. While the water is present, decompositional and ecological processes such as aquatic invertebrate scraping and shredding can occur. Other processes such as photodegredation and mechanical breakdown of debris by terrestrial species become the primary mechanism of decomposition during dry periods (Collins //et al.// 2008). The mosaic of conditions within intermittent rivers create reservoirs of nutrients that are often re-distributed after a secondary wetted front initiates.

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">//**Biodiversity**//

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Historically, intermittent rivers were perceived to be outside the scope of both aquatic or terrestrial ecologists, leaving them to be largely overlooked. It was also been assumed that the relatively low biodiversity within intermittent systems was dominated by drought-tolerant species (Lytle and Poff, 2004). However, research on temporary streams has increased within the last decade and studies are showing that the habitat mosaics created by the dynamic transitions between aquatic and terrestrial environments have the ability to support unique and diverse biota (Datry //et al.,// 2010).

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">The shifting availability of habitat types created by flow variability can be classified as aquatic, terrestrial, or transitional. The natural cycles of drying and rewetting require species to display a variety of life history adaptations to ensure their survival. Species exhibit differing behavioral, morphological, and physiological characteristics such as encysting invertebrates and estivating fish that can survive numerous wet/dry periods for months or years (Larned //et al.// 2010). Biota tend to inhabit the system after the initial pulse has passed and a lower, more stable flow exists. The temporary lower flows can provide critical habitat for species of fish whose life strategy requires periodic low flows. There is always a risk of exposure for the eggs if the flows discontinue suddenly, or risk of stranding for both adult and/or juvenile fish by the river desiccation events.

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;"> Wading birds, fish, aquatic and terrestrial invertebrates, salamanders, turtles, coyotes, toads, songbirds, snakes, deer, algae, aquatic macrophytes, nearby terrestrial vegetation, protozoans, fungi, and more can all use the habitat provided by intermittent rivers throughout some portion of its expansion-contraction cycle. Whether the species uses the channel as a source of drinking water, a breeding ground, or for the entirety of its existence the number of biota that use intermittent rivers is greater than originally believed. <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Figure 3 to the right demonstrates the relationship between the percentage of annual flow intermittence and the number of taxa found within the system. According to the figure, there appears to be a trend relating the presence of more species with less intermittence. The intermittent rivers used to create the figure are shown below. <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;"> a) Albarine River, France - benthic invertebrates (Datry 2012) <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;"> b) Selwyn River, New Zealand - benthic invertebrates (Arscott et al. 2010) <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;"> c) Selwyn River - hyporheic invertebrates (Datry et al. 2007); <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;"> d) San Pedro River, Arizona - riparian plants (Stromberg et al. 2005) <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;"> e) Selwyn River - fish (Davey and Kelly 2007)

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Figure 4 below shows the responses exhibited by aquatic and terrestrial organisms throughout habitat inundation and drying periods within intermittent systems

<span style="display: block; font-family: Tahoma,Geneva,sans-serif; font-size: 110%; text-align: right;">Figure 3: Datry //et al.// (2014) <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 110%;"> Figure 4: Larned et al. 2010

//**Anthropogenic and climate change impacts**//
<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Global water resources are experiencing tremendous pressure from anthropogenic water abstraction, land use changes, and climate change effects. Intermittent river systems are particularly vulnerable because they lack adequate management practices and protective policies. The most common threats to intermittent rivers are surface and groundwater pumping, impoundments, and flow manipulations. The same threats apply to many perennial systems which are often converted to intermittent systems as a result of these anthropogenic impacts. Many of these intermittent induced situations are characterized by quick changes from continually lotic systems to periodic zero flow conditions resulting in serious ecological consequences. Climate induced intermittency is also an issue, however it is likely to occur at a slower rate that is in tandem with regional drying trends (Larned //et// //al//., 2010). Southern Africa, Europe, the Amazon and western North America are predicted to experience a 2-3 fold increase in the recurrence of low flows, which are also associated with a 25-45% decrease in flow according to climate runoff models (Arnell, 2003).

//**Restoration & Management**//
<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;"> Intermittent rivers are in need of their own unique management separate from perennial systems. Traditional management practices for continually flowing systems are not applicable to intermittent rivers because of their naturally shifting lotic, lentic, and terrestrial habitat mosaics. The recent increase in temporary river research is encouraging, however the integration of science from traditionally separate disciplines would lead to advances in knowledge and improvement in management practices.

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">These three restoration objectives as identified by Larned et al. 2010, are necessary to provide to managers as a basis for the most essential criteria needed to maintain intermittent river health. <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">1. Restoration of aquatic-terrestrial mosaics <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">2. Preservation or restoration of natural flow regimes <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">3. Identify flow requirements for species of value and ecological properties

<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Figure 5 lists several methods and concepts that can be applied to the restoration of intermittent rivers. <span style="display: block; font-family: Tahoma,Geneva,sans-serif; font-size: 120%; text-align: right;">Figure 4: Datry et. al (2014) //**<span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Conclusion **// <span style="font-family: Tahoma,Geneva,sans-serif; font-size: 120%;">Intermittent rivers are widespread throughout the world. Their role as a source of biodiversity and provider of ecosystem services makes them highly valued parts of the global ecology. "They are critical conduits for water, energy, material, and organisms even when surface water is not present (Acuna //et. al,// 2014)". These systems are being degraded at alarming rates due to development, water abstraction, and climatic changes. The lack of legislative protection for these valuable systems needs to change, and with it new management strategies for these unique ecosystems needs to emerge.

<span style="display: block; font-family: Tahoma,Geneva,sans-serif; font-size: 110%; text-align: left;">//**Sources**// <span style="font-family: Tahoma,Geneva,sans-serif;">Acuna V., Datry T., Marshall J., Barcelo D., Dahm C.N., Ginebreda A., McGregor G., Sabater S., Tockner K. & Palmer M.A. (2014) Why should we care about temporary waterways? //Science,// **343**, 1080-1081.

Arscott D.B., Larned S.T., Scarsbrook M.R. & Lambert P. (2010) Aquatic invertebrate community structure along an intermittence gradient: Selwyn River, New Zealand. //Journal of North American Benthological Society//, **29**, 530-545.

<span style="font-family: Tahoma,Geneva,sans-serif;">Austin A.T., Yahjian L., Stark J.M., Belnap J., Porporato A., Norton U., Ravetta D.A. & Schaffer S.M. (2004) Water pulses and biogeochemical cycles in arid and semiarid ecosystems. //Oceologia//, **142**, 221-235.

<span style="font-family: Tahoma,Geneva,sans-serif;">Bull, W.B. (1997) Discontinuous ephemeral streams. //Geomorphology//, **19**, 227-276

<span style="font-family: Tahoma,Geneva,sans-serif;">Collins, S.L., Sinsabaugh R.L., Crenshaw C., Green L., Porras-Alfaro A., Stursova M. & Zeglin L.H. (2008) Pulse dynamics and microbial processes in arid land ecosystems. //Journal of Ecology//, **96**, 413-420.

<span style="font-family: Tahoma,Geneva,sans-serif;">Datry, T., Larned, S.T. & Tockner K. (2014) Intermittent Rivers: a challenge for freshwater ecology, //BioScience//, **64**, 299-235.

<span style="font-family: Tahoma,Geneva,sans-serif;">Davey A.J.H. & Kelly D. (2007) Resistance and resilience of fish communities to drying disturbances in an intermittent stream: a landscape perspective. //Freshwater Biology//, **52**, 1719-1733.

<span style="font-family: Tahoma,Geneva,sans-serif;">Larned, S.T., Datry, T, & Robinson C.T. (2007) Invertebrate and microbial responses to inundation in an ephemeral river reach in New Zealand: effects of preceding dry periods. //Aquatic Sciences: Research Across Boundaries//, **69**, 554-567.

<span style="font-family: Tahoma,Geneva,sans-serif;">Larned, S.T., Datry T., Arscott, D.B. & Tockner, K. (2010) Emerging concepts in temporary-river ecology. //Freshwater Biology//, **55**, 717-738.

<span style="font-family: Tahoma,Geneva,sans-serif;">Lytle D.A. & Poff N.L. (2004) Adaptation to natural flow regimes. //Trends in Ecology and Evolution//, **19**, 94-100.

Meybeck M., Laroche L., Durr H.H. & Syvitski J.P.M. (2003) Global variability of daily suspended solids and their fluxes in rivers. Global and Planetary Change, **39**, 65-93.

<span style="font-family: Tahoma,Geneva,sans-serif;">McIntyre R.E.S., Adams M.A., Ford D.J. & Grierson P.F. (2009) Rewetting and litter addition influence mineralization and microbial communities in soils from a semi-arid intermittent stream. //Soil Biology and Biochemistry//, **41**, 92-101.

<span style="font-family: Tahoma,Geneva,sans-serif;">Palmer M.A., Liermann C.A.R., Nilsson C., Florke M., Alcamo J., Lake P.S. & Bond N., (2008) Climate change and the world's river basins: anticipating management options. //Frontiers in Ecology and the Environment//, **6**, 81-89.

<span style="font-family: Tahoma,Geneva,sans-serif;">Patton P.C. & Schwumm S.A. (1980) Ephemeral stream processes: Implications for studies of quaternary valley fills. //Quatenary Research//, 15**,** 24-43.

<span style="font-family: Tahoma,Geneva,sans-serif;">Power M.E., Sun A., Parker G., Dietrich W.E. & Whootton J.T. (1995) Hydraulic foodchain models. //Bioscience//, **45**, 159-167

<span style="display: block; font-family: Tahoma,Geneva,sans-serif; font-size: 90%; text-align: left;">Schwinning S. & Sala O.E. (2006) Hierarchy of responses to resource pulses in arid and semi-arid ecosystems, //Oecologia//, **141**, 211-220

Sheldon F., Bunn, S.E., Hughes J.M., Arthington A.H., Balcombe S.R. & Fellows C.S. (2010) Ecological roles and threats to aquatic refugia in arid landscapes: Dryland river waterholes, //Marine and Freshwater Research//, **61**, 885-895.

Snelder T.H., Datry T., Lamouroux N., Larned S.T., Saquet E., Pella H. & Catalogne C. (2013) Regionalization of patterns of flow intermittence from gauging station records. //Hydrological Earth System Sciences//, **17**, 2685-2699.

Stromberg J.C., Bagsted K.J., Leenhoust J.M., Lite S.J. & Making E. (2005) Effects of stream flow intermittency on riparian vegetation of a semiarid region river (San Pedro, Arizona). //River Research and Applications//, **21**,925-935.

Tooth S. (2000) Splay formation along the lower reaches of ephemeral rivers on the Northern Plains of arid central Australia. //Journal of Sedimentary Research//, **75**, 636-649.