Category: News

Tonle Sap Lake is unique in its ecological and cultural importance. In the rainy season, water flows upstream from the Mekong River to swell the lake to 4 times its dry-season extent, creating extensive shallow water habitat for fish. When the lake drains back to the Mekong through the Tonle Sap River, it creates one of the greatest fishing grounds of the world, providing protein to millions of people. Research by former Riverlab/Fulbright visiting scholar Chantha Oeurng and his student Ty Sok demonstrates evolving flow and sediment relations between the lake and the Mekong River. The just-published paper,  Assessment of Suspended Sediment Load Variability in the Tonle Sap and Lower Mekong Rivers, Cambodia, is available for free download until late May from Catena here.

Two Riverlab researchers (Rafael Schmitt and Matt Kondolf) and two Italian colleagues (Simone Bizzi and Andrea Castelletti) have been awarded the 2021 Aspen Institute Italia Award for their joint research on “Improved trade-offs of hydropower and sand connectivity by strategic dam planning in the Mekong” ^[1].  The scientists are working on this ongoing effort to reduce impacts of the global clean energy transition on rivers and livelihoods.

Building future green economy poses great economic and technical challenge for societies, in part because of the often-overlooked externalities of technology and infrastructure on people and the environment. In their research, the scientists used the example of hydropower development to demonstrate need and opportunity to resolve such conflicts through strategic spatial planning. Hydropower is a well-proven and cost-effective way to generate renewable energy. At the same time, dams can have catastrophic impacts on people’s livelihoods and the fundamental processes that underpin healthy rivers. Thus, there is great concern about the environmental impacts of future dams, mostly planned to energize socio-economic development in the global south.

The winning research demonstrated that strategic placement of dams, considering for the spatial heterogeneity of natural processes in rivers and the cumulative impacts of multiple dams, can greatly reduce dam impacts without compromising on energy generation and energy costs. That finding was derived by combining a novel computer model for evaluating large scale impacts of dams on rivers with state-of-the art tools for decision analysis. The study was based on the example of the Mekong River in South East Asia, where a massive hydropower development occurred in the recent past, and more development is foreseen in the near future. Results show that existing dams, exploiting around 50 % of the basin’s hydropower potential, have major impacts on the biophysical functioning of the studied rivers. The key finding of the study is that the same amount of hydropower could have been generated with much smaller impacts if dam sites would have been selected strategically such as to reduce their cumulative impacts.

While the results were derived for the Mekong River, the findings have broad implications for renewable water and energy systems world-wide. Globally, increasing conflicts between infrastructure and natural systems are inevitable: Other forms of renewable energy create environmental impacts, more water infrastructure will be required to meet domestic and agricultural water demands, and industrialized countries must soon review their aging infrastructure portfolios. In this context, strategic decision making, which balances economic and ecosystem needs is crucial for an ecologic transition to water and energy systems with minimal impacts on nature and maximal benefits for society.  

---

[1]  The reasearch has been published by Nature Sustainability | VOL 1 | FEBRUARY 2018 | 96–104 |

Deliberate high-flow releases are increasingly made from dams to mimic effects of floods and interact with the channel to produce biophysical changes in channel characteristics, such as removing fine sediment from downstream aquatic habitats. These are a special case of environmental flows intended to mitigate geomorphic/ecological effects of dams, commonly termed flushing flows. In a new publication, Remi Loire and colleagues propose new terms for these high flow releases: morphogenic releases, or ecomorphogenic releases for flows intended specifically to improve aquatic and riparian habitats. The paper, published in Earth Science Reviews, reviews objectives of these flows, experiences gained from their implementation, and potential conflicts with environmental, socio-economic, and dam-operational issues. The paper is available until 21 February 2021 for free download here.

Loire, R, H Piégay, J-R Malavoi, GM Kondolf, and LA Bêche. From flushing flows to eco-geomorphic flow releases: evolving terminology, practice, and integration into regulated river management. Earth Science Reviews 213: 103475

Many cities worldwide now have available waterfront land, often in the city center, thanks to de-industrialization and concentration of navigation elsewhere. As cities seek to take advantage of this remarkable real estate, they may be prone to some classic ‘traps’ such as copying projects successful in another city but which fail in the new location due to differences in scale, topography, urban form, etc. In this recent publication, Pedro Pinto and Matt Kondolf present an idiosyncratic list of ‘wrongs’ that are evident in many such projects. The paper is freely available, via open access here.

Pinto, JP, and GM Kondolf. 2020. The fit of urban waterfront interventions: matters of size, money and function. Sustainability 12: 4079; doi:10.3390/su12104079

River restoration projects in North America that involve reconfiguration of stream channels are dominated by symmetrical, single-thread meandering channels.  Although the meander dimensions are commonly justified by relations between channel width and meander wavelength, the universal preference for single-thread meandering channels in restoration projects is rarely questioned.  The aesthetic appeal of s-shaped curves in art and landscape design may help explain the prevalence of this form in river restoration projects. Riverlab alumna Kristen Wilson (Nature Conservancy) gave 300 freshwater scientists attending her keynote talk 5 minutes to draw a restored stream.  She compiled the results to see what mental images these scientists had for restored channels. Most depicted single-thread meanders for their restored channel, although there were interesting variants.  See Kristen’s just-published paper here.

 

Throughout the humid tropics, increased land disturbance and concomitant road construction increases erosion and sediment delivery to rivers.  Building road networks in developing countries is commonly a priority for international development funding based on anticipated socio-economic benefits. Yet the resulting erosion from roads, which recent studies have shown result in at least ten-fold increases in erosion rates, is not fully accounted for.  While effects of road-derived sediment on aquatic ecosystems have been documented in temperate climates, little has been published on the effects of road-induced sediment on aquatic ecosystems in developing countries of the tropics.  Along the south bank of the Rio San Juan (Nicaragua and Costa Rica), attempts to build a road without engineering or plans resulted in massive failures and erosion in areas where steep slopes impinge upon the river bank.  Pre-existing tributary streams received elevated sediment loads, creating new deposits on pre-existing tributary deltas.  In some reaches with rapidly eroding sites, completely new deltas of freshly deposited sediment were formed, prograding into the river channel.

Riverlab alumni Blanca Rios and Scott Walls joined with Matt Kondolf to study periphyton biomass and macroinvertebrate communities on the deltas of Río San Juan tributaries, comparing north-bank tributaries draining undisturbed rain forest with south-bank tributaries receiving runoff from the partially-built road experiencing rapid erosion. Periphyton biomass, richness and abundance of macroinvertebrates overall, and richness and abundance of Ephemeroptera, Plecoptera and Trichoptera were higher on the north-bank tributary deltas than the south-bank tributary deltas. These findings were consistent with prior studies in temperate climates showing detrimental effects of road-derived fine sediment on aquatic organisms.  A Non-Metric Multidimensional Scaling (NMDS) analysis showed the impacted community on the south-bank deltas was influenced by poorly-sorted substrate with greater proportions of fine sediment and higher water temperatures.  The paper is freely available (open-access) here.

Rios-Touma, B, GM Kondolf, and SP Walls. 2020. Impacts of sediment derived from erosion of partially-constructed road on aquatic organisms in a tropical river: the Río San Juan, Nicaragua and Costa Rica. PLoSONE 15(11):e0242356. https://doi.org/10.1371/journal.pone.0242356

We are excited to share a new Riverlab article, linked here, published this month in the Journal of Hydrology. This work advances current understanding of the controls on hydrological connectivity of impervious surfaces to downstream channels and storm-sewer networks and presents new methods of their estimation.

Connected impervious areas – those impervious surfaces that contribute directly to runoff in a storm network or stream – are a better indicator of hydrologic response, stream alteration, and water quality than total impervious area. Most methods for quantifying connected impervious areas require major assumptions regarding the definition of ‘connection’, potentially over-simplifying the role of variable climates, slope gradients, soils conditions, and heterogeneous flow paths on impervious surface connectivity.

In this study, we present a new metric, hydrologically connected impervious areas (HCIA), to refer to spatially explicit (mapped) estimates of the proportion of impervious surfaces that are hydrologically connected to the storm sewer system or stream network. HCIA is comprised of impervious surfaces that contribute directly to the storm-sewer network and are physically connected, A_phys, or those that contribute indirectly and are therefore variably connected (A_var) (see Figure 1). The degree to which A_var is “hydrologically connected” is represented with a coefficient, ϕ_var, that ranges between 0 and 1, with 0 representing full connectivity (i.e. all runoff infiltrates downslope), and 1 representing no connectivity (i.e. no runoff infiltrates downslope).

Using a combination of hydrologic modeling in the PySWMM, a python interface for the EPA’s Stormwater Management Model, and machine-learning regression tree analysis, we evaluate the controls on ϕ_var across varing soil types, slopes, rainfall scenarios, antecedent soil moisture conditions, as well as amounts of impervious and pervious areas. Figure 6 shows that of the factors tested, soil texture (panel A), fraction of downslope pervious area ϕ_perv (panel B), soil moisture (panel C), and precipitation (panel D) are sensitive, while total area (panel D), width of impervious area (panel F), and slope (panel G) are insensitive parameters.

To assist with dissemination of these methods in practice, we apply the regression tree in a geospatial tool for estimation of HCIA in ungauged urban catchments. We test the tool in a case study to an urban sewershed in Colorado, and find that the contribution of A_var to HCIA (compared to the contribution of A_phys) varied across the precipitation and soil moisture conditions. A_var contribution to HCIA was low at low precipitation depths and increased rapidly with increasing precipitation and initial soil moisture conditions (see Figure 9).

• 

Overall, our results suggest that, for catchments consisting of highly impermeable soils, A_var contributes to HCIA such that HCIA approaches the total impervious area, but for catchments with highly permeable soils, A_var does not contribute significantly to HCIA, and thus the physically connected impervious area ( A_phys) could be used as a suitable surrogate for HCIA. In between these two extremes, however, lies a wide range of conditions that call for detailed and spatially explicit estimates of A_var connectivity.

References Cited

Sytsma, A., Bell, C., Eisenstein, W., Hogue, T., & Kondolf, G. M. (2020). A geospatial approach for estimating hydrological connectivity of impervious surfaces. Journal of Hydrology, 591, 125545. https://doi.org/10.1016/j.jhydrol.2020.125545 >>link to paper

Our Riverlab events have been cancelled for Spring 2020 due to COVID-19.

You may be interested in a new analysis of the effects of recently completed Xayaboury Dam (on the Mekong mainstem near Luang Prabang) and a cascade of dams upstream in China on flow patterns in the Lower Mekong River. The paper, Mekong River, Xayaboury Dam, and Mekong Delta in the first half dry season 2019-2020 by Nguyen Ngoc Tran was published in Vietnamese in TIA SANG, a scientific journal published by the Ministry of Science and Technology. The English version is now available here. As illustrated in excerpts of Figures 5 and 6 from the paper, the hydrologic analysis shows that flows this dry season have been significantly lower than in prior years’ dry seasons.

Riverlab members have contributed scientific papers on the cumulative effects of upstream dams on the sediment budget of the Mekong Delta and other threats to the sustainability of the Delta (Kondolf et al 2014, Kondolf et al 2018) and the potential for strategic dam planning to minimize impacts of dams on downstream sediment budgets and fish migration (Schmitt et al 2019).

Water levels in the Mekong River at Nakhon Phanom reflecting severe drought conditions in the current dry season of the 2019-2020 water year. (Source: Nguyen Ngoc Tran. 2020, Mekong River, Xayaboury Dam, and Mekong Delta in the first half dry season 2019-2020, Figure 5.)

View of the exposed bed of the Mekong River at Nakhon Phanom in late October 2019, reflecting severe drought conditions in this year’s dry season. (Source: Nguyen Ngoc Tran. 2020, Mekong River, Xayaboury Dam, and Mekong Delta in the first half dry season 2019-2020, Figure 6.)

References Cited

Kondolf, G.M., Z.K. Rubin, J.T. Minear. 2014. Dams on the Mekong: Cumulative sediment starvation.  Water Resources Research 50, doi:10.1002/2013WR014651. >>link to paper

Kondolf, GM, RJP Schmitt, P Carling, S Darby, M Arias, S Bizzi, A Castelletti, T Cochrane, S Gibson, M Kummu, C Oeurng, Z Rubin, and T Wild. 2018. Changing sediment budget of the Mekong: Cumulative threats and management strategies for a large river basin. Science of the Total Environment 625: 114-134. https://doi.org/10.1016/j.scitotenv.2017.11.361 >>link to paper

Schmitt, R, S Bizzi, AF Castelletti, J Opperman, GM Kondolf. 2019. Planning dam portfolios for low sediment trapping shows limits on sustainable hydropower in the Mekong. Science Advances 5: eaaw2175 >>link to paper