Non-perennial segments in river networks

Thibault Datry, Andrew Boulton, Ken Fritz, Rachel Stubbington, Nuria Cid,
Julie Crabot, Klement Tockner

► To cite this version:

Thibault Datry, Andrew Boulton, Ken Fritz, Rachel Stubbington, Nuria Cid, et al. Non-perennial segments in river networks. Nature Reviews Earth & Environment, 2023, 4 (12), pp.815-830. 10.1038/s43017-023-00495-w. hal-04444942

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1 Why non-perennial segments must be included in river science and management

2 Thibault Datry¹†, Andrew J. Boulton², Ken Fritz³, Rachel Stubbington⁴, Nuria Cid⁵,⁶, Julie Crabot⁷,

3 Klement Tockner⁸

4 ¹INRAE, UR RiverLy, Centre Lyon-Grenoble Auvergne-Rhône-Alpes, 5 rue de la Doua CS70077,

5 69626 Villeurbanne Cedex, France, ORCID: 0000-0003-1390-6736

6 ²Ecosystem Management, School of Environmental and Rural Science, University of New England,

7 Armidale, 2350, New South Wales, Australia, ORCID: 0000-0001-7393-2800

8 ³Office of Research and Development, United States Environmental Protection Agency, 26 West

9 Martin Luther King Drive, Cincinnati, Ohio 45268 USA, ORCID: 0000-0002-3831-2531

10 ⁴School of Science and Technology, Nottingham Trent University, Nottingham NG11 8NS, UK,

11 ORCID: 0000-0001-8475-5109

12 ⁵IRTA Marine and Continental Waters Programme, Ctra de Poble Nou Km 5.5, E43540, La Ràpita,

13 Catalonia, Spain

14 ⁶FEHM-Lab (Freshwater Ecology, Hydrology and Management), Departament de Biologia

15 Evolutiva, Ecologia i Ciències Ambientals, Facultat de Biologia, Universitat de Barcelona (UB),

16 Diagonal 643, 08028 Barcelona, Spain, ORCID: 0000-0002-9997-5523

17 ⁷Université Clermont Auvergne, CNRS, UMR GEOLAB, F-63000 Clermont-Ferrand, France,

18 ORCID: 0000-0002-7864-993X

19 ⁸Senckenberg Society for Nature Research and Faculty of Biological Sciences, Goethe-University,

20 Frankfurt a. M., Germany, ORCID: 0000-0002-0038-8151

21 †e-mail: [thibault.datry@inrae.fr](mailto:thibault.datry@inrae.fr)

22
23 Abstract

24 Non-perennial river segments that recurrently cease to flow or typically dry occur in all river
25 networks and, globally, are more abundant than perennially flowing segments. Their biodiversity
26 and ecosystem processes are governed by the repeated cycles of flowing, non-flowing and dry
27 phases that, in turn, influence the ecosystem dynamics in connected perennial segments and
28 downstream waters. In this Review, we outline how the flow regime in non-perennial segments,
29 especially the dry phase, influences their biodiversity and ecosystem processes at different spatial
30 scales, from individual segments to entire river networks. Varying configurations of perennial and
31 non-perennial segments govern each river network's physical, chemical and ecological responses to
32 changes in flow regimes, especially in response to human activities. We discuss the likely future of
33 non-perennial segments from hydrological and ecological perspectives, then review the management
34 implications of the connections between perennial and non-perennial segments. We conclude by
35 suggesting five specific and actionable recommendations for research within the next 3-5 years to
36 clarify how non-perennial segments influence connected perennial waters and what this means for
37 effective management at the river network scale. Neglecting the important roles of non-perennial
38 streams compromises holistic river management.

39 **Key points**

40 1. All river networks have naturally non-perennial segments. These segments constitute over half the global river network length. Ongoing climate change, land-use alteration and overexploitation will further increase the occurrence of river drying in both space and time.

43 2. Recurrent cycles of flowing, non-flowing and dry phases influence the supplies of water, energy,  
44 nutrients and organisms that non-perennial segments exchange with connected perennial waters
45 3. Physical, chemical and biological processes in non-perennial segments affect water quality and quantity, and ecological integrity, in downstream receiving waters and entire river networks.

46 4. Historically, river science and management have focused on perennial river segments, neglecting the ubiquity and importance of non-perennial segments. This imbalance has often led to environmental problems such as poor water quality and loss of natural flow regimes at the river network scale.

51 5. Sustaining the water quality and ecological integrity of entire river networks and associated downstream waters requires integrated management strategies that explicitly consider non-perennial segments and their connections with perennial ones.

54
55 **Keywords:** Biodiversity, catchment, connectivity, drying, ecological function, ecosystem service, ephemeral stream, hydrological phase, intermittent river, river network, water management, water policy.

58
59 **Introduction**
60 Although rivers cover less than 2% of the Earth's surface, they contain approximately 13% of all described species and provide key ecosystem services such as provision of drinking water and food, regulation of climate and opportunities for recreation¹,². However, rivers are among the world's most threatened ecosystem types; one out of three species in these biodiversity hotspots is threatened with extinction³. River science and management have historically focused on perennial segments, which flow year round, but rivers are increasingly being conceptualized as spatially
variable networks in which hydrological connections between perennial and non-perennial segments (Box 1) support network-scale biodiversity and ecosystem functioning⁴,⁵ (Figure 1a).

Non-perennial river segments (hereafter NPRs) recurrently cease to flow and most lose most or all surface water (Figure 1c). They include ‘ephemeral’ or ‘intermittent’ river segments (Box 1), with ‘ephemeral’ implying shorter and less-predictable flowing phases than ‘intermittent’, but no agreed distinction exists. Despite efforts to classify river segments in which flows cease for varying periods and with varying predictability⁶⁻⁹, global consensus remains elusive because of the high temporal and spatial variability within and among NPRs¹⁰. Moreover, variability will further increase in the future. In this Review, we refer to non-perennial rivers and segments, following Messager *et al.*⁴.

Every river network encompasses NPRs, particularly in the headwaters (Figure 1a), but sometimes also substantial lengths of the lower segments. NPRs naturally constitute more than half of the global river network length⁴ (Figure 1b), a proportion that is expected to rise in some regions because of climate change, land-use alteration and increased water abstraction¹¹,¹².

When flowing (Figure 1c), NPRs supply water, aquatic biota, energy, nutrients and other materials to connected waters, influencing their water quality, biodiversity and ecological integrity. These pulsed hydrological connections are often irregular in time and space yet generate dynamic transition zones between aquatic and terrestrial habitats that extend longitudinally down river channels, laterally onto floodplains and vertically into the underlying groundwaters¹³.

Despite the ubiquity of NPRs and the ecological importance of this hydrological connectivity, most policies and management practices for river networks are tailored for rivers that are perennial. The omission of NPRs from river management seriously risks undermining effective protection of biodiversity and ecological integrity of entire river networks and their ecosystem services. Scientists
88 and managers need greater awareness of the importance of NPRs to biodiversity and ecosystem processes in river networks.

90 In this Review, we explore key insights from research on NPRs in the context of network-scale riverine connectivity. For brevity, we focus on aquatic responses to drying from segment to river network scale but acknowledge that interactions with terrestrial ecosystems are also crucial in NPRs, especially during dry phases. We conclude by exploring the drawbacks of excluding NPRs from river network management. Our central message is that the current scientific conceptualization of river networks as arrays of co-occurring perennial and NPRs must be matched with changes to river management that recognize the essential importance of integrating NPRs into strategies and actions that protect and, where possible, enhance network-scale biodiversity, ecosystem processes and ecosystem services.

99 **1. Characterizing non-perennial segments**

100 NPRs are common across river networks. We now discuss their flow regimes, the types of connections between perennial segments and NPRs in river networks, their distribution within different networks and how such connections can alter fluxes of water, materials and organisms across river networks.

104 **1.1 Typology and connections**

105 *Flow regimes*

106 The flow regime, defined by the temporal variability in the quantity and timing of discharge, governs river geomorphology, water quality and ecology¹⁴ and, thus, is crucial to the understanding and management of river networks. In NPRs, the flow regime encompasses alternating flowing,
109 non-flowing and, in many cases, dry phases (Figure 1c, Box 1). The frequency, duration and timing
110 of each phase are powerful determinants of biodiversity, ecological integrity and ecosystem
111 services, both at local scales within NPRs¹⁵,¹⁶ and at the river-network scale¹⁷,¹⁸.

112 Drying (Box 1) is a gradual process whose effects are separated by hydrological thresholds¹⁹. Early
113 in the process while flow declines, surface water contracts and lateral aquatic habitats become
114 disconnected. As drying progresses, riffles and other flowing surface habitats disappear, leaving the
115 riverbed as disconnected pools (Figure 1c). Eventually, the riverbed dries, although subsurface flow
116 may continue in saturated sediments (the hyporheic zone) below the dry channel¹⁹⁻²¹ (Figure 1c).

117 This loss of surface water is governed by the type of surface-groundwater interactions occurring at
118 the segment scale²²⁻²⁴. When segments are losing surface water, the water table of the underlying
119 aquifer is generally deep and the hyporheic zone usually dries quickly²⁰,²⁴, whereas in gaining
120 segments, upwelling groundwater can maintain a saturated hyporheic zone throughout the dry
121 phase²⁰,²⁴.

122 During rewetting (Box 1), surface water returns to inundate the dry channels, sometimes quickly as
123 flash floods²⁵,²⁶ or slowly as a rise in water table. Like all components of flow regimes, the
124 characteristics of drying and rewetting transitional phases are driven by both natural climatic and
125 geological factors and by human activities²⁷⁻²⁹ and usually vary greatly within and between river
126 networks³⁰,³¹. The consequences of such variability remain poorly understood but are likely to be an
127 important determinant of a river network's biodiversity and ecological integrity¹⁸,³².

128 *Perennial and non-perennial segment connections*
129 Most NPRs are structurally connected to perennial segments by the spatial continuity of the fluvial corridor (i.e., the channel, hyporheic and floodplain³³) (Figure 1a), reflecting their ubiquitous co-occurrence in river networks globally (Figure 1b). This connectivity is well captured by the meta-ecosystem perspective that considers river networks as mutually dependent arrays of perennial and NPRs that are connected to adjacent aquatic and terrestrial ecosystems³⁴. The connected terrestrial ecosystems range from riparian zones and uplands in the headwaters to floodplains downstream, and the connected aquatic ecosystems encompass adjacent wetlands, lakes, subsurface groundwaters, downstream reservoirs, estuaries and coastal waters (Figure 1a).

137 The spatial arrangements of perennial and NPRs and their connections are diverse, complex and dynamic (Figure 2 a&b). Most river networks have naturally non-perennial headwaters that span the interface between terrestrial and aquatic domains³⁵. Flow in headwater segments is driven by a combination of surface runoff, groundwater inputs and/or meltwater and is often seasonal¹⁷,³⁶. NPRs can also be in the downstream parts of river networks (Figure 2b). Typically, river networks that flow out onto porous alluvial plains and recharge the groundwater below have downstream NPRs, such as the Tagliamento River (Italy²⁵) and the Albarine River (France³⁷). Other causes of downstream NPRs include excessive evaporation such as in the Diamantina River (Australia³⁸) or human activities such as damming and water abstraction in the Colorado River (US) or Yellow River (China³⁹). In other river networks, the upper and lower segments are perennial but the mid-segments are non-perennial, such as in the Selwyn River (New Zealand⁴⁰). In arid regions, whole river networks are often non-perennial, such as many rivers in northern and southwestern Africa⁴¹ and the Nordeste in Brazil⁴². The varying physical settings and flow regimes of these different network configurations drive physical, chemical and ecological responses to the connections between NPRs and perennial segments.
152 Functional connections among river segments

153 The different functional roles played by the varying connections between perennial and NPRs in
154 space and time can be classified by the general mechanisms by which stream segments influence
155 fluxes to downstream waters<sup>43</sup> (<strong>Figure 2c, Supplementary 1</strong>). When fluxes of water, sediments,
156 material and organisms pass from perennial to NPRs and <em>vice versa</em>, they are often altered, which
157 may have contrasting effects on downstream river segments (<strong>Figure 2c</strong>). These variable effects
158 occur because fluxes of material or energy can cease, increase, decrease or be stored when they pass
159 through NPRs before entering downstream waters<sup>43</sup>. Consequently, the functional connections
160 between perennial and NPRs usually modify the quantity and quality of material and energy moving
161 through the river networks.

162 Connections between perennial and NPRs can act as sources of material, notably when the organic
163 material accumulated during dry phases is leached during rewetting, causing high nutrient fluxes to
164 downstream waters<sup>44</sup> (<strong>Figure 2c</strong>). When NPRs dry, the exchange of water ceases, both vertically
165 between surface and groundwaters and longitudinally from upstream to downstream waters,
166 transforming NPRs into sinks of material<sup>45</sup> (<strong>Figure 2c</strong>). As dry phases progress, terrestrial leaf litter
167 from adjacent riparian zones is retained in NPRs and gradually accumulates at the bed surface<sup>32</sup>
168 (<strong>Figure 2c</strong>). Upon rewetting, this organic material is flushed downstream, sometimes massively<sup>26</sup>,
169 with NPRs functioning to delay its release (<strong>Figure 2c</strong>). NPRs also offer refuges during the dry phase
170 for organisms including microbes, invertebrates and fish<sup>46,47</sup> (<strong>Figure 2c</strong>). Lastly, connections
171 between perennial and NPRs can transform spatial patterns in chemical parameters such as dissolved
172 oxygen when anoxic or hypoxic pulses of water from remnant pools or rewetting fronts are carried
173 downstream to connected aquatic ecosystems by rewetting flows<sup>18,48</sup> (<strong>Figure 2c</strong>). The types and
direction of processes that are altered are likely to reflect flow regime characteristics such as dry phase duration and the longitudinal sequence of perennial and NPRs.

## 1.2 Global distribution and temporal trends

NPRs abound on all continents, representing more than half of the global river network⁴ (Figure 1b). For example, 94% of river lengths in southwestern USA⁴⁹ and more than 70% of river lengths in Australian rivers are non-perennial⁵⁰. NPRs typically dominate in arid, semi-arid, and dry sub-humid regions, which represent up to half of the Earth's land surfaces³⁹. These segments are also common across alpine, boreal, continental, Mediterranean, oceanic, polar and tropical regions¹⁵,⁵¹.

Every river network on Earth includes NPRs, especially their headwater streams, which are estimated to represent more than 70% of the total river network length and are typically prone to drying⁴,³⁵.

## 2. The ecology of non-perennial segments

Drying in NPRs acts as a master variable that controls local biotic communities, ecosystem processes and ecosystem services. In addition, the spatial arrangement and type of connections between perennial and NPRs at the river-network scale shape the ecological integrity of river networks. In this section, we discuss how drying influences the ecology of NPRs at the segment scale and how these effects propagate across the entire river network.

### 2.1. Non-perennial segment scale

From microorganisms to vertebrates and higher plants, all biotic groups respond to recurrent shifts from flowing to non-flowing and dry conditions in NPRs (Figure 1c). Across biotic groups, aquatic biodiversity declines steadily in response to non-perenniality as taxa lacking adaptations promoting resistance or resilience to drying are lost¹⁵,¹⁶. The extent of these declines is governed by
hydrological parameters such as the duration of non-flowing and dry phases, with longer dry phases resulting in greater declines³⁷,⁴⁶,⁵². For example, a ten-day increase in the dry phase in the Albarine River, France, led to an additional loss of six taxa from the benthic invertebrate community and four taxa from the hyporheic zone³⁷. An increase in the duration of the non-flowing phase from 0 to 78 days reduced the survival of Coho salmon populations from 0.59 to 0.11 from tributaries of the Russian River in California⁴⁶. Concurrent increases in terrestrial species richness occur as colonists arrive from riparian and wider terrestrial habitats⁵³. For example, 22 and 12 invertebrate taxa colonized the dry riverbeds of the Albarine River (France) and Oaky Creek (Australia), respectively, within two months of the onset of the dry phase⁵³. Although flowing-phase aquatic species richness at the site-scale can be considerably lower in NPRs than perennial segments⁵⁴, their contribution to regional aquatic-terrestrial biodiversity can exceed those of perennial segments⁵¹,⁵⁵.

When flow resumes, aquatic organisms recolonize from various refuges, including upstream pools, moist sediments and leaf litter⁵⁶⁻⁵⁸. Some aquatic macroinvertebrates and diatoms have desiccation-tolerant forms that can survive in moist riverbed sediments⁵⁹,⁶⁰. Recovery rates and trajectories vary depending on connectivity to refuges. Communities in NPRs connected to upstream perennial waters are soon replenished by colonists⁶¹ whereas community recovery in isolated NPRs may be slower and more stochastic⁵⁷,⁶². Recovery can be modified by anthropogenic influences such as fragmentation by instream barriers, which sever links between NPRs and sources of colonists in perennial segments⁶³. Despite well-developed recovery mechanisms, differences typically persist between communities in perennial and non-perennial segments³⁷,⁵⁴. These differences can be particularly pronounced and long-lasting when unprecedented dry phases occur during drought events. In contrast, communities can recover rapidly, generally within a few weeks, after seasonal dry phases⁶²,⁶⁴.
219 Aquatic species carry out many fundamental ecosystem processes, and changes in aquatic
220 biodiversity related to non-perenniality therefore alter ecosystem functioning. Desiccation-tolerant
221 microorganisms at the base of food webs in natural NPRs mediate many biogeochemical functions.
222 When flow decreases, hypoxic conditions that develop in disconnected pools favor denitrification,
223 reducing nitrate concentrations²⁴. During dry phases, the microorganisms that make up the biofilms
224 coating the surfaces of sediment particles emit large quantities of CO₂⁶⁵ and upon rewetting, massive
225 CO₂ pulses can occur from NPRs. Accounting for the global prevalence of NPRs, a single rewetting
226 event can contribute up to 10% of the daily carbon dioxide emissions from all perennial rivers and
227 streams, particularly in temperate climates³². Recognizing the active contribution of NPRs to carbon
228 cycling – during both wet and dry phases – could improve the accuracy of local-to-global-scale
229 assessments⁶⁶. When water returns, ecosystem processes such as respiration⁶⁷,
230 nitrification/denitrification⁶⁸ and decomposition of leaf litter⁶⁹ quickly resume.

231 Due to drying-driven decreases in the functional diversity of their aquatic communities, NPRs can
232 collectively perform fewer ecosystem functions than perennial ones during flowing phases⁷⁰. These
233 declines may be mitigated by functional redundancy⁷¹ (multiple species sharing traits), making the
234 drying-induced loss of individual species functionally inconsequential⁷². But as losses accumulate,
235 the risk of losing a functionally unique species increases, potentially representing a tipping point that
236 drives the ecosystem to an alternative state. For example, by eliminating pivotal species, especially
237 predators, drying can alter the structure and functioning of food webs, potentially leading to partial
238 food-web collapse⁷³. Similarly, the loss of desiccation-sensitive microorganisms and invertebrate
239 detritivores reduces decomposition rates of leaf litter and other particulate organic matter that fuels
240 some food webs⁷⁴, altering the quantity, quality and timing of energy sources transported
241 downstream to perennial segments after flow resumes⁴⁴,⁷⁵.
242 **2.2. River-network scale**

243 The logical extension of how drying shapes the ecology of NPRs is to explore how the collective
244 effects of responses to segment-scale drying extend to river networks. Network-scale responses to
245 drying are unlikely to be simple additive effects of segment-scale responses, but are more likely a
246 result of synergistic effects of responses to segment-scale drying as they propagate through the
247 network.

248 Firstly, spatial and temporal patterns of drying vary among river networks³¹,⁷⁶,⁷⁷ (Figure 2b).

249 Longitudinal trends related to elevation and channel form are superimposed on idiosyncratic drying
250 patterns, thwarting efforts to extrapolate segment-scale patterns or assume that ‘river continuum’
251 predictions are realistic¹⁷,¹⁸,⁴⁰.

252 Secondly, surface and subsurface flows propagate biogeochemical and ecological influences
253 downstream during flowing phases but these transfers typically slow, cease or are amplified by
254 functional connections across hydrological phases that are likely to lead to ‘hotspots’ and ‘hot
255 moments’ of material processing along a river network²⁴. For example, terrestrial plant litter
256 accumulates in dry and non-flowing segments and this litter decomposes very slowly³² (Figure 3).

257 When flow resumes in these segments, it can trigger rapid microbial decomposition of the labile
258 plant litter³² and flush the decomposing litter downstream²⁶ to perennial segments as a delayed
259 subsidy of a resource that may be limiting to downstream consumers⁷⁸ (Figure 3). However, the
260 network-scale effects of non-perenniality largely depend on the spatial arrangement of NPRs within
261 the river network (Figure 3). Where NPRs are concentrated in the headwaters, in particular in
262 deciduous forested areas³⁵, the downstream effects of non-perenniality will be high seasonal inputs
263 of unprocessed litter (Figure 3) that can reduce downstream water quality⁷⁹ or cause technical
264 problems for dam intakes⁸⁰. In contrast, where NPRs are in downstream sections of river networks,
265 which are typically more autotrophic and less dependent on terrestrial litter inputs from adjacent
266 riparian zones, lower fluxes of unprocessed litter are expected in downstream river network
267 segments while headwaters could become hotspots of carbon cycling (Figure 3).

268 Synchrony (the degree of concurrent change across spatially distinct segments or populations) can
269 describe how local responses propagate at the river network scale. Synchrony can be enhanced by
270 connectivity (upstream dispersal, advective transport) between segments or when a driver such as
271 drying simultaneously affects multiple segments<sup>81</sup>. By reducing longitudinal connectivity of flowing
272 water, the non-perenniality of river segments can reverberate throughout a river network and
273 desynchronize, for example, diel dissolved oxygen fluctuations <sup>18</sup> or the recovery of biological
274 communities during flowing phases<sup>17</sup>. When flow resumes in NPRs, hydrological connectivity is
275 restored, promoting network-scale synchronization of such fluctuations and processes<sup>18</sup>.

276 Widespread non-perenniality in river networks can also synchronize dynamics across populations,
277 reducing regional metapopulation stability and increasing the risk of regional extinctions<sup>82,83</sup>. The
278 dynamics of aggregate stream systems are typically less variable than their individual contributing
279 segments<sup>84,85</sup> because combining variable, asynchronous contributions from many segments has a
280 stabilizing effect (the ‘portfolio concept’<sup>84</sup>). Therefore, when aggregated in river networks, the
281 variation in flow regimes between perennial and NPRs contributes to persistence of regional
282 biodiversity and thus stable ecosystem functioning and associated availability of ecosystem
283 services<sup>83</sup>.

284 At the river-network scale, NPRs substantially increase the beta diversity (regional variability in
285 community composition between locations and/or times) of aquatic communities, because of the
286 simultaneous coexistence of different successional stages at the river network scale<sup>17,39,86</sup> (Figure 4).
287 Depending on the spatial arrangement of NPRs and perennial segments within the river network,
288 communities in the former can comprise a subset of the taxa inhabiting the latter, notably when
289 NPRs are downstream of perennial segments acting as a source of coloniits¹⁵ (Figure 4).

290 Alternatively, when non-perenniality is concentrated upstream, the composition of their biological
291 communities may be more variable in space or time compared to their downstream perennial
292 counterparts<sup>86,87</sup> (Figure 4). This variability is because recolonization from downstream perennial
293 segments is limited by the directional flow down river networks, promoting alternative sources of
294 colonists in the landscape such as perennial waterbodies and the underlying hyporheic zone.

295 This integration of NPRs into biodiversity assessment of river networks reveals the importance of
dispersal, a regional process which can dominate over the local process of species-sorting during
296 rewetting phases<sup>17,88,89</sup>. The spatial extent of drying influences the access to and from refuges in the
297 network during the dry periods and thus controls the dispersal of organisms and overall
298 metacommunity dynamics<sup>17,90,91</sup>. However, the relative contribution of species dispersal in shaping
299 riverine biodiversity patterns is highly context-dependent and extends to adjacent habitats such as
300 riparian zones<sup>56</sup>, hyporheic zones and groundwaters<sup>92</sup> and downstream ecosystems<sup>93</sup>.
301

## 2.3 The future of river networks

### 303 Changes to non-perennial flow regimes: widespread and increasing

304 The spatial extent of NPRs in global river networks has increased since 1980<sup>11,94,95</sup> (Figure 5a & b).

305 This increase is predicted to continue<sup>12,42,96</sup>, driven in part by climatic trends such as rising
temperatures and associated increases in evaporation, changing precipitation patterns, and the
306 increasing occurrence of drought in many parts of the world<sup>97-99</sup>. For example, previously perennial
307 rivers in Europe and China dried for the first time during the severe droughts that began in
308 2022<sup>100,101</sup>. In addition, intensifying use of water resources, including surface and groundwater
309 abstraction, storage and diversion, are driving shifts from perennial to artificially non-perennial
310
311 flow³⁹,¹⁰⁰. For example, in Kansas, USA, widespread irrigation from the High Plains aquifer since
312 the mid-1900s has lowered the regional water table, sometimes by more than 50 m, shifting rivers
313 and streams from naturally gaining and perennially flowing to artificially losing and non-perennial
314 flow¹⁰²,¹⁰³.

315 The predicted increase in the spatial extent of NPRs represents a network-scale decline in aquatic
316 habitat availability and hydrological connectivity which will alter riverine biodiversity, with
317 consequences for ecosystem functioning and services. For example, a decrease in the availability
318 and quality of wet refuges that support aquatic organisms during dry phases will synchronize
319 biological responses to drying, reducing metacommunity resilience and local community recovery
320 after flow resumes⁸³.

321 Dry phases are getting longer, drier and more frequent

322 Flow regime characteristics such as the frequency, duration, severity and timing of flowing, non-
323 flowing and dry phases are also changing, as is the rate of change between transitional drying and
324 rewetting phases. Despite considerable regional variability in the evidence for such changes to
325 date¹¹,⁹⁵ (Figure 5a & b), there is high confidence that changes will occur in the near future. These
326 predicted changes include increases in dry-phase duration and, based on studies of future drought
327 characteristics, increases in dry-phase frequency, severity⁹⁸ and rates of onset of both dry phases and
328 flow resumption¹⁰⁴. Inherent within the predicted increase in dry-phase duration is the earlier onset
329 and/or later cessation of dry phases in NPRs with seasonal flow regimes, as well as an increased co-
330 occurrence of dry phases and extreme climatic events, particularly heatwaves¹⁰⁵⁻¹⁰⁷.
331 In the context of emerging megadroughts<sup>108,109</sup>, dry phases could continue uninterrupted for years in
332 NPRs that currently have seasonal flow regimes. Deglaciation and snow loss are expected to reduce
333 summer flows, resulting in shifts to non-perennial flow<sup>110,111</sup>. Conversely, warmer winters with
334 greater snowmelt and glacial melting could cause NPRs to become perennial at northern latitudes<sup>42</sup>.

335 **Biological responses to future changes in non-perennial flow regimes**

336 How will biological communities in NPRs respond to future changes in flow regime characteristics?
337 An increase in dry-phase duration and severity (caused by a lack of precipitation and/or high
338 temperatures and manifesting as reduced in-channel water availability) could reduce the survival of
339 desiccation-tolerant life stages of aquatic organisms within the riverbed sediments<sup>59</sup>. An increase in
340 dry-phase duration and frequency in NPRs could eliminate desiccation-sensitive species without
341 enabling colonization by tolerant equivalents<sup>71</sup> (Figure 5c). Faster wet-to-dry transitions could
342 shorten the time between environmental cues that trigger insect metamorphosis and its completion,
343 reducing the emergence of adults<sup>112</sup>. Rapid-onset rewetting phases that wash insects, crustaceans,
344 amphibians or fish straight from refuges to downstream segments could reduce local community
345 recovery rates<sup>113-115</sup>. Both earlier dry-phase onset and later dry-phase termination could reduce
346 successful completion of aquatic stages of life-cycles by riverine animals. For example, earlier onset
347 of drying could reduce the proportion of aquatic juvenile insects that emerge as terrestrial adults in
348 time to avoid a dry phase<sup>112</sup>. Later dry-phase termination could prevent egg-laying behaviors by
349 species that oviposit on water<sup>116</sup>. Distances and connectivity to perennial refuges in the landscape
350 may determine post-drying community composition<sup>83,117</sup>. In cases of shifts from perennial to non-
351 perennial flow regimes, biological responses might be particularly dramatic because species may
352 lack adaptations to drying. However, if NPRs are abundant in a river network, they may provide
353 colonists adapted to the newly non-perennial conditions<sup>83,100</sup>.
In terms of ecosystem processes, biofilms generally recover within a few days upon flow resumption, from dormant forms and through drift¹¹⁸. Some ecosystem processes, such as primary production or ecosystem respiration, are therefore highly resilient to drying. As such, natural NPRs experiencing longer or more frequent dry periods might not be severely affected in the near future, although shifts towards greater heterotrophy could occur¹¹⁹,¹²⁰. Other processes could be more dramatically affected by increasing drying, such as the decomposition of terrestrial leaf litter, because they are more dependent on macroorganisms¹⁰⁰. In situations where once-perennial segments become non-perennial, biodiversity is predicted to respond strongly, with multiple cascading effects on ecosystem processes, although these effects will depend on the functional redundancy of a community and the types of organisms involved¹⁰⁰,¹¹⁹,¹²¹. Finally, where increasing non-perenniality is coupled with other human-induced stressors (channelisation, contamination, water abstraction), how ecosystem processes respond to drying could be highly complex.

In most cases, a reduction in aquatic biodiversity is the initial ecological effect of predicted future increases in drying. These taxonomic changes have functional consequences, altering ecosystem processes and associated ecosystem services. Although biological communities have, to date, proven resilient to drying, recovering within weeks to a few years even from rare dry phases in NPRs¹²²,¹²³, predicted future changes in riverine flow regimes have increasing potential to tip ecosystems to new, functionally compromised stable states. For example, decreases in flowing-phase duration and frequency could interact with concurrent stressors such as artificial enrichment by inorganic nutrients, to shift aquatic vegetation communities from habitat-forming plants to filamentous algae. This change would alter basal food resources and habitat availability for invertebrates and fish, triggering trophic cascades that extend through food webs. In addition, plants act as ecosystem engineers that alter sediment dynamics, and their loss could thus alter river shape¹²⁴,¹²⁵. In contrast,
377 the ecological consequences of region-specific shifts towards perenniality could increase network-scale hydrological connectivity, promoting biotic dispersal and thus homogenizing communities<sup>89,126,127</sup>.

380 **3. Non-perenniality in river network management**

381 The effects of alternating flowing, non-flowing and dry phases on water quality, ecosystem processes, biodiversity and ecosystem services at the river network scale mean that management expectations of naturally NPRs must differ from those of perennial segments<sup>92,128,129</sup>. In particular, effective management, conservation and restoration of river networks must explicitly recognize perennial and NPRs and their connections. Below, we discuss management implications generated by the connections between perennial and NPRs and how management should account for NPRs in river networks.

388 **3.1 Management implications of non-perenniality**

389 Understanding the connections between perennial and NPRs, and with other connected waters (lakes, reservoirs, wetlands, aquifers, estuaries and in coastal areas) is a crucial step towards integrated management of river networks. The processes enabled by these connections have major, network-scale implications for biodiversity conservation, water quality management, risk mitigation such as floods and the availability of ecosystem services.

394 Dry-phase refuges located in NPRs are crucial to maintain freshwater biodiversity at the river network scale<sup>46,130</sup>. For example, isolated pools maintained in intermittent streams in coastal Oregon<sup>131</sup> and in the Russian River in California<sup>46</sup> during dry phases provide refuges that promote the survival of juvenile Coho salmon (*Oncorhynchus kisutch*) (Figure 6a). In the Russian River, the mean cumulative survival of salmon in these pools reached 50%<sup>46</sup>. The deterioration or loss of these
habitats, together with the lack of access to them due to fragmentation by anthropogenic barriers¹³²,
reduces fish populations and thus impacts subsistence fishers⁴⁶. Management efforts for biodiversity
conservation may require adaptation to prioritize the identification and protection of these
refuges¹³³,¹³⁴.

NPRs can attenuate floods and act as flood protection zones in the catchment (Figure 6b). For
example, the dry channel of the ephemeral river Rambla de Nogalte, SE Spain, efficiently absorbed
flash flood waters and sediments – except where walls and embankments had been built¹³⁵.

Infiltration potential (the extent to which water can enter the sediments) in NPRs should be
integrated in flood risk management at the river network scale by identifying priority zones where
this function is maximized. The high infiltration capacity of dry riverbeds in NPRs can limit
evaporative losses and facilitate groundwater recharge (Figure 6c). For example, flooding after a
rain event in the ephemeral Sand River, Kenya, recharged the groundwater level in only 1.5
hours¹³⁶. Similarly, 49% of the monsoon flood volume from ephemeral Río Puerco basin, New
Mexico, USA, recharged the aquifer and the rest entered a downstream reservoir⁴⁵. The resultant
availability of water resources can be a major benefit for people living and depending on these water
resources in arid and semi-arid regions.

Nutrients released from NPRs during flowing phases can subsidize downstream connected waters
and support biodiversity and ecosystem functioning downstream, enhancing services provided by
freshwater and marine fisheries (Figure 6d). For example, the timing of a nutrient pulse from the
ephemeral Santa Clara River to at least 20 km from the California coast during the 1998 floods was
key to supporting marine productivity at a time when nutrient inputs from oceanic upwelling were
less available¹³⁷.
421 Non-perenniality has major consequences for provisioning, regulating and cultural ecosystem services made available by river networks. River drying prevents surface water abstraction for uses including crop irrigation and public water supply¹³⁸. However, in many regions, a substantial fraction of public water supply comes from NPRs¹³⁹ or underlying groundwaters¹⁴⁰. Surface water loss is also likely to reduce fish populations that support subsistence fishers⁴⁶. Regulating services such as water purification, flood mitigation and climate regulation are all compromised by drying¹⁴¹.
422
423 For example, drying eliminates desiccation-sensitive microorganisms from biofilms and slows assimilation of inorganic nutrients (including anthropogenic pollutants) after flow resumes¹⁴².
428
429 Drying also affects cultural services of river networks by limiting water-associated activities such as boating while creating opportunities for new activities such as rambling¹³⁸,¹⁴³,¹⁴⁴. These impacts on cultural services depend on people's perceptions of drying¹⁴⁵,¹⁴⁶ which has a major effect on how river networks with extensive NPRs are likely to be managed.
430
431
432

### 3.2 Improving river network management

433
434 Human activities alter flow regimes in NPRs with major implications for functional connections and adjacent perennial segments. However, compared to perennial segments, these NPRs are seldom as well-protected by legislation from the impacts of human activities that might alter their flow¹⁰⁰,¹⁴⁷ (Figure 7) because management practices have yet to be adapted to match conceptual developments in river science that recognize the ecological importance of non-perenniality. Such adaptation in management practices would necessarily extend to other connected waters, such as floodplain wetlands, lakes and alluvial groundwaters, as well as the estuaries and near-shore coastal waters associated with river networks that drain to the sea.
435
436
437
438
439
440
441

442
443 River network management can be improved by applying insights from scientific research demonstrating the implications of the functional connections between perennial and NPRs in
444 spatially variable river networks. For example, if the legislative protection of perennial segments in
445 many countries were automatically extended to NPRs and their catchments, it could likely reduce
446 the impacts of human activities that threaten biodiversity and ecosystem function of river networks
447 and their connected ecosystems (Figure 7). Some nations already have limited legislative protection
448 for NPRs. In Australia, independent scientific assessment of the potential environmental impacts of
449 large coal mining and coal seam gas developments on water-dependent biota and ecosystems in
450 perennial segments and NPRs in the disturbance footprint is expressly required under legislation (the
451 2013 'water trigger' amendment of the Environment Protection and Biodiversity Conservation Act
452 1999<sup>148</sup>). This example, from a single country, demonstrates how legislation can protect valued
453 natural assets in both perennial and non-perennial waterbodies.

454 Major obstacles need to be overcome to extend legislative protection for NPRs<sup>147,149</sup>. In Europea,
455 NPRs are largely omitted from conservation, restoration and biomonitoring<sup>153</sup> by the Water
456 Framework Directive (WFD)<sup>150</sup>. For example, the WFD only recognizes NPRs in one 'river type' in
457 some countries in the Mediterranean region. Elsewhere, where NPRs are less prevalent but
458 nonetheless diverse and extensive such as France<sup>151</sup>, the UK<sup>55</sup> and the Czech Republic<sup>152</sup>, river
459 typologies used to implement the WFD do not distinguish between perennial and NPRs. In some
460 cases, such as France, there are attempts to remove NPRs from national? legislation (Figure 7h).
461 Similarly, in the US, the Clean Water Act does not specify NPRs as Waters of the United States,
462 exposing them to impacts such as dredging and waste dumping.

463 Another obstacle is the public perception of NPRs in river networks as less valuable than flowing
464 waters<sup>145,146,154</sup>. Consequently, NPRs are often overlooked in restoration and conservation plans<sup>146</sup>
465 and there is little appreciation of their ecosystem services<sup>141,144</sup>. Even in Australia, where the need
466 for separate water quality guidelines for NPRs is accepted<sup>155</sup>, there is limited appreciation of the
467 importance of connectivity between perennial and NPRs in influencing water quality. Scientists 468 need to communicate the importance of ecosystem services provided by NPRs to the general public, 469 river managers, politicians, policy makers and other stakeholders. Such evidence-informed actions 470 may include producing fact sheets and policy briefs, using social media and conducting 471 collaborative research projects with citizen scientists, river managers and stakeholders. For example, 472 the open-source smartphone application DryRivers enables both citizen and professional scientists to 473 map NPRs throughout Europe¹⁵⁶ and has substantially increased public appreciation of the extent of 474 NPRs.

475 Logistically, there are serious limitations in fundamental hydrological data and hydrographic 476 mapping for NPRs in river networks. Stream gauges that quantify flow and describe flow regime 477 components of a stream segment are typically placed along larger perennial streams and fail to 478 capture flow regimes of NPRs¹⁵⁷. Most available maps are based on static, low-resolution surveys 479 and cartography that omit many headwater NPRs³⁵,¹⁵⁸. Despite increasing efforts at various scales to 480 statistically ⁴,¹⁵⁹⁻¹⁶¹ and mechanistically model¹⁶²,¹⁶³ distributions and flow regimes of NPRs, such 481 efforts are hampered by the scant stream gauge and groundwater-level data, which amplifies 482 uncertainty over large spatial extents ⁴,¹⁶¹. Further development and refinement of multi-platform 483 remote-sensing technology could be combined with modeling approaches that target stream gauging 484 or field observations to reduce bias and fill gaps, enabling the production of hydrographic maps that 485 better reflect the dynamic connections between all segments in river networks.

486 Another obstacle to more effective management of connected perennial and NPRs in river networks 487 is the limited availability of tools and approaches that perform well in both segment types. As in 488 river sciences, most river management tools have been developed primarily or exclusively for 489 perennial segments¹²⁹,¹⁶⁴,¹⁶⁵ and typically perform poorly in NPRs. For example, biomonitoring
indices used to indicate river health are rarely able to disentangle the effects of drying from those of stressors associated with human activities¹²⁹,¹⁶⁴,¹⁶⁶,¹⁶⁷. Functional traits¹⁶⁷, metasystem approaches¹⁶⁸, molecular tools¹⁶⁹ and terrestrial and semi-aquatic species¹⁷⁰,¹⁷¹ have the potential to enhance assessment of NPR health. Developing common tools and approaches that are equally effective in both perennial and NPRs would encourage river managers to include both types of segments in river network biomonitoring. For some NPRs, specialized tools may not be required¹²⁹,¹⁷² (but see ⁹²,¹⁶⁶,¹⁷⁰); effective biomonitoring could be achieved by timing monitoring periods to coincide with flowing phases, and by setting appropriate expectations of the values of biomonitoring indices used to indicate ecological integrity, to match those in natural NPRs rather than perennial ones.

500

## 4. Summary and future perspectives

Repeated cycles of flowing, non-flowing and, in particular, dry phases govern the biodiversity and ecosystem processes of NPRs, in turn influencing ecosystem dynamics in connected perennial segments and downstream waters. NPR can function as sources, sinks and refuges for water, energy, nutrients and organisms, and can delay and transform such ecosystem components, thus governing their fluxes across these connections. We contend that scientific recognition of the importance of these hydrological connections between perennial and NPRs in spatially variable river networks must be matched with a shift in river management. To facilitate such a shift, policy developments are needed to extend the legislation that protects perennial rivers to include NPRs.

We identify five specific and actionable research within the next 3-5 years to further characterize how NPRs influence connected perennial segments and what this means for effective management at the river network scale:
513 1. Identify how different functional connections affect fluxes of materials, organisms and water
514 from NPRs to perennial segments in different parts of river networks (e.g. upstream
515 headwaters vs downstream segments). One approach is to use experimental manipulations to
516 demonstrate causal mechanisms. Such experiments should explore multiple fluxes
517 concurrently due to their interacting effects.

518 2. Characterize how physical and hydrological features influence different types of fluxes. For
519 example, how different are fluxes and their effects in small ephemeral NPRs compared to
520 larger intermittent ones.

521 3. Evaluate how the collective effects of different functional connections affect ecosystem
522 services provided by whole river networks. For example, lagged connections may nullify or
523 delay the influence of other types of connections upstream and alter the types, timing and
524 location of ecosystem services contributed by NPRs.

525 4. Analyze how human activities modify functional connections in different river networks and,
526 in particular, impact the provision of valued ecosystem services. For example, the clearance
527 – or restorative planting – of riparian vegetation along NPRs may change downstream
528 functional connections and fluxes of organic matter.

529 5. Use the scientific evidence provided by such research to inform management actions as well
530 as policy developments that enhance holistic legislative protection for NPRs within river
531 networks, that might help prevent the ongoing losses of biodiversity and ecological functions
532 occurring in river networks worldwide.

533 Sustaining the quality of entire river networks and downstream waters, including rivers, lakes,
534 reservoirs, groundwaters, estuaries and in coastal areas, requires integrated management strategies
535 that consider NPRs and their interconnections with perennial waters. Neglecting the important roles
536 of NPRs compromises successful holistic river management and ignores crucial insights provided by
537 the recent paradigm shift in river science.

538

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955

956 Acknowledgements

957 TD received support from the European Union’s Horizon 2020 research and innovation programme
958 through the DRYvER project (Securing Biodiversity, Functional Integrity, and Ecosystem Services
959 in Drying River Networks, award number 869226). The views expressed in this manuscript are
960 those of the authors and do not necessarily reflect the views or policies of the USEPA.

961

962 Competing interests

963 We have no conflict of interest to declare.

964

965 Author contributions

966 TD, AJB, KF: conceptualization, Writing- Original draft preparation. JC, KT, NC, RS: Writing-
967 Original draft preparation. All: Writing- Reviewing and Editing,

968

969

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971

972

973

974
975 **Box 1: Glossary**

976 **Dry phase.** In a non-perennial river segment, a period of time with no spatially continuous flowing or non-flowing surface water, although isolated surface-water pools and subsurface water may be present.

979 **Drying phase.** In a non-perennial river segment, the transitional period between a flowing or non-flowing phase and a dry phase, during which most or all surface water is lost

981 **Ephemeral.** A non-perennial flow regime in which water only flows in response to rainfall events, and flowing phases are thus unpredictable and typically short (hours to weeks).

983 **Flow cessation.** The point in time at which surface water ceases to flow from upstream to downstream in a non-perennial segment.

985 **Flowing phase.** In a non-perennial river segment, a period of time in which water flows from upstream to downstream.

987 **Intermittent.** A non-perennial flow regime, often seasonal, that is typically characterized by long flowing phases (usually multiple months) and shorter dry phases.

989 **Non-flowing phase.** In a non-perennial river segment, a period of time when spatially continuous non-flowing (still or lentic) surface water is present.

991 **Non-perennial.** Non-perennial river segments are those in which surface water recurrently stops flowing. These segments lose all or most of their surface water.

993 **Perennial.** Perennial river segments are those in which surface water never stops flowing.

994 **Rewetting phase.** In a non-perennial river segment, the transitional period between a dry phase and a flowing or non-flowing phase.
<figure><img src="image_2.png" /></figure>

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996

997

<caption>Figure 1. Non-perennial river segments: definition, abundance and flow regimes. a, An idealized river network, indicating the different types of non-perennial (ephemeral and intermittent) river segments and their linkages with adjacent waters. b, Global prevalence of non-perennial river networks (from Messager *et al.*⁴). c, A typical hydrological sequence of a non-perennial segment, from flowing, non-flowing, dry and rewetting phases in the Calavon River, France (photos: B. Launay).</caption>

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<figure><img src="image_8.png" /><figcaption>Figure 2. The connections between non-perennial and perennial river segments. a. Examples of connections between perennial and non-perennial segments, T Datry. b. River-network patterns of co-occurring non-perennial (dashed lines) and perennial (solid lines) segments. c. Functions affecting fluxes of water, material and organisms through non-perennial segments (turquoise triangle) before entering downstream waters (gray triangle). Changes in arrow thickness reflect changes in the fluxes through the functional connections. Changes in arrow color reflect conversion of material or energy form. Changes in arrow shape reflect delayed delivery of material or energy. After Leibowitz et al.<sup>43</sup>.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 3. Effects of non-perenniality on river-scale leaf litter decomposition and transport.</figcaption></figure>

Two theoretical river networks with contrasting spatial arrangement of non-perennial
segments, upstream (a) and downstream (b). Leaf litter is poorly decomposed during dry
phases in non-perennial segments. Instead, it accumulates and is then transported downstream
en masse when flow resumes. As such, the fluxes of decomposed and undecomposed litter vary
substantially between the two river networks. Modified from Datry *et al*¹⁰.
<figure><img src="image_10.png" /><figcaption>Figure 4: Consequences of non-perenniality for biodiversity patterns at the river network scale. Expected diversity patterns of two hypothetical river networks in which non-perennial sections are located upstream (a) and downstream (b). Circles represent communities hosting different species (symbols). For the two river networks, the magnitude of the effect of drying on biodiversity is the same (the same number of species disappears along a longitudinal gradient of drying). When non-perenniality is concentrated in the headwaters, the species-poor communities are composed of species not found in species-rich communities in</figcaption></figure>
1038 downstream perennial segments. Conversely, when non-perenniality occurs downstream, the
1039 species-poor communities of these segments are subsets of species-rich communities from
1040 upstream perennial segments. These network patterns emerge because recolonization is faster
1041 from upstream to downstream, following the directionality of river flow.

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<figure><img src="image_11.png" /><figcaption>Figure 5; The future hydrological and biological fate of non-perennial rivers. a. Increasing (upward triangle) or decreasing (downward triangle) trends at the European scale (cross indicate no trends), at the 10% significance level, for the annual mean number of zero-flow⁹⁵. b. Mann–Kendall trends in annual no-flow days across the USA; red indicates longer no-flow duration¹¹. c. Relationships between annual no-flow duration (as a percentage) and the taxonomic richness (in the number of taxa) of lotic communities. The letter labels mark (a) benthic invertebrates in the Albarine River, France³⁷; (b) benthic invertebrates in the Selwyn River, New Zealand⁴⁰; (c) hyporheic invertebrates in the Selwyn River¹⁷³; (d) riparian plants in the San Pedro River, Arizona¹⁷⁴; and (e) fish in the Selwyn River¹⁷⁵. The lines are based on regression models published in the original studies.</figcaption></figure>

<figure><img src="image_12.png" /></figure>
<figure><img src="image_13.png" /><figcaption>Figure 6: Examples of ecosystem processes and services occurring in non-perennial segments and management opportunities. ⁴⁵,¹³¹,¹⁷⁶ a. Residual/isolated pools in a NP segment from Oregon¹³¹ and Coho salmon juveniles in a NP segment from the Russian River in California (USA). Photo: Russian River Monitoring Program. b. Rambla de Nogalte (SE Spain) during a flood in 2012, and the risk innundation map. Source: Confederación Hidrográfica del Segura. c. The Puerco River, a tributary of Río Grande (USA) during the flowing period, and the comparison of flood hydrographs and well response during 2003 floods⁴⁵. Photo: USGS.</figcaption></figure>
1086 d. Satellite image of Santa Clara ephemeral river inputs to the Channel Islands in California

1087 (USA) during 1998 floods. River mouth and mid-channel surface water observations of

1088 nutrients and chlorophyll-a during these floods¹³⁷. Arrows show the river mouth. Photo:

1089 Landstat.

1090

1091

1092

1093

1094
<figure><img src="image_14.png" /><figcaption>Figure 7: Examples of threats on non-perennial river segments. a. garbage in the dry riverbed of the Hodgsons Creek, Victoria, and b. in Madura gully, West Australia (A. Boulton) c. a non-perennial segment of the Chitterne Brook flows through an intensively grazed cow pasture in England, UK (T. Sykes); d. sheep in the non-flowing segments of the Barranc del Carraixet, Spain (N. Cid), e. Sewage effluent turning the non-perennial segment of the Sant Miquel River artificially perennial in Spain (N. Cid), f. & g. gravel extractions from dry riverbeds in France (Albarine River) and Bolivia (Janq'u Qala) (T. Datry), h. An example of a map showing in red, the NPRs to be removed from protection by legislation in France, on the</figcaption></figure>
1104 basis of their non-perenniality. The perennial segment in blue is the only part of the river
1105 network under protection by law. i. sewage effluent release generates a permanent pool in a
1106 non-perennial segment of the Albarine River, France (T. Datry); and j. example of a NPRs
1107 that is no longer under protection in eastern France, le Ruisseau des Tendasses (H. Pella).

1108
1109
1110 Supplementary 1: Videos illustrating how dynamic the connections between non-perennial
1111 and perennial segments can be.

1112
1113 https://www.youtube.com/watch?v=ccAUgBZCxY8

1114 https://youtu.be/_e2R_l8Xfi0

1115 https://www.youtube.com/watch?v=fi7wwLCCj0c

1116 https://www.youtube.com/watch?v=ORJtxkuD62E

1117 https://www.youtube.com/watch?v=aZp_1KtrzjQ

1118 https://www.youtube.com/watch?v=KZocXIEgwS8

1119 https://twitter.com/LewisPugh/status/1651555685253935104