Philipp Theuring

Suspended sediments in the Kharaa River catchment (Mongolia) and its impact on hyporheic zone functions

A previous study investigating the ecological status of the Kharaa River in Northern Mongolia reported fine-grained sediments as being a major stress factor causing adverse impacts on the benthic ecology. However, the source of these sediments within the catchment as well as the specific impact on hyporheic zone functions in the Kharaa River remained unclear. Therefore, the objective of the current study was to investigate the underlying source–receptor system and implement an integrated monitoring approach. Suspended sediment sources within the Kharaa catchment were identified by using extensive spatially distributed sediment sampling and geochemical and isotope fingerprinting methods. On the receptor side, the ecological implications across a gradient of fine-grained sediment influx were analyzed using a distinct hyporheic zone monitoring scheme at three representative river reaches along the Kharaa River. Results of suspended sediment source monitoring show that during snowmelt runoff, riverbank and gully erosion were the dominant sources. During the summer period, upland erosion contributed a substantial share of suspended sediment. Fine-grained sediment influx proved to be the cause of habitat loss in the hyporheic zone and benthic oxygen production limitation. This combined catchment and in-stream monitoring approach will allow for a better understanding and spatially explicit analysis of the interactions of suspended sediment transport and hyporheic zone functioning. This information has built the basis for a coupled modeling framework that will help to develop efficient management measures within the Kharaa River basin with special emphasis on rapidly changing land-use and climatic conditions.

INTEGRATING MULTI-SCALE DATA FOR THE ASSESSMENT OF WATER AVAILABILITY AND QUALITY IN THE KHARAA—ORKHON—SELENGA RIVER SYSTEM

The environmental and socio-enonomic impacts of water pollution are particularly severe in regions with relatively limited water resources [WWAP, 2012]. Water quantity and quality are closely interlinked aspects which are relevant for surface water ecology, water use, and integrated management approaches. However, an intensive monitoring of both is usually prohibitive for very large areas, particularly if it includes the investigation of underlying processes and causes. For the Kharaa - Orkhon - Selenga River system, this paper combines results from the micro (experimental plots, individual point data), meso (Kharaa River Basin) and macro (Selenge River Basin) scales. On the one hand, this integration allows an interpretation of existing data on surface water quantity and quality in a wider context. On the other hand, it empirically underpins the complimentary character of intensive monitoring in selected model regions with more extensive monitoring in larger areas.

Source identification of fine-grained suspended sediment in the Kharaa River basin, northern Mongolia

Fine sediment inputs into river systems can be a major source of nutrients and heavy metals and have a strong impact on water quality and ecosystem functions of rivers and lakes, including those in semiarid regions. However, little is known to date about the spatial distribution of sediment sources in most large scale river basins in Central Asia. Accordingly, a sediment source fingerprinting technique was used to assess the spatial sources of fine-grained (<10 μm) sediment in the 15 000 km(2) Kharaa River basin in northern Mongolia. Variation in geochemical composition (e.g. in Ti, Sn, Mo, Mn, As, Sr, B, U, Ca and Sb) was used for sediment source discrimination with geochemical composite fingerprints based on Genetic Algorithm (GA)-driven Discriminant Function Analysis, the Kruskal-Wallis H-test and Principal Component Analysis. All composite fingerprints yielded a satisfactory GOF (>0.97) and were subsequently used for numerical mass balance modelling with uncertainty analysis. The contributions of the individual sub-catchment spatial sediment sources varied from 6.4% (the headwater sub-catchment of Sugnugur Gol) to 36.2% (the Kharaa II sub-catchment in the middle reaches of the study basin), generally showing higher contributions from the sub-catchments in the middle, rather than the upstream, portions of the study area. The importance of river bank erosion is shown to increase from upstream to midstream tributaries. The source tracing procedure provides results in reasonable accordance with previous findings in the study region and demonstrates the applicability and associated uncertainties of the approach for fine-grained sediment source investigation in large scale semi-arid catchments.

Cause–effect–response chains linking source identification of eroded sediments, loss of aquatic ecosystem integrity and management options in a steppe river catchment (Kharaa, Mongolia)

Although sparsely populated, the progressive degradation of Mongolia’s rivers, lakes and groundwater, driven by land-use changes, poses a key challenge for the future sustainable development of the country. This paper deciphers the cause–effect–response chain between river bank degradation, changes of the ecological status, declines of ecosystem functions and priority measures with the case of the Kharaa River in Northern Mongolia. The underlying research approach comprised: (1) hydromorphological characterisation of the Kharaa River, (2) water quality assessments, (3) determination of the riverbed composition including hyporheic zone properties, (4) the analysis of riverine biota (macroinvertebrates and primary producers) and (5) the identification of the sources of suspended and settled sediments. The assessment revealed a gradient of spatially heterogeneous river bank erosion due to the degradation of the riparian vegetation caused by overgrazing and wood utilization. As the most prominent ecological response, the biomass of benthic algae decreased and macrozoobenthic community metrics changed continuously along the pressure gradient, accompanied by shifts of habitat related functional traits. At the same time, the hyporheic zone dimensions and functioning were affected by suspended and infiltrated sediments in multiple ways (restricted spatial extent, lowered hydraulic connectivity, lower metabolism, ecologically critical quality of pore water). Geochemical and radionuclide fallout isotope fingerprinting has identified riverbank erosion as the main source of the suspended sediments in the Kharaa River, when compared to gully and land surface erosion. Erosion susceptibility calculations in combination with suspended sediment observations showed a strong seasonal and annual variability of sediment input and instream transport, and a strong connection of erosional behaviour with land-use. Amongst others, the protection of headwaters and the stabilization of the river bank erosion hotspots in the mid-stream sections of the Kharaa River are the priority measures to avoid further degradation of the aquatic ecosystem status and functions.