Formation of high water turbidity in small tidal estuaries of the White and Barents Seas

The formation of increased turbidity zones in tidal estuaries is an important factor influencing their hydrochemical, hydrobiological and hydroecological regimes. The authors solved the problem of identifying regularities for processes in the estuarine areas of small rivers flowing into the tidal sea and analyzed the features caused by morphological patterns and the tide magnitude. Expeditionary studies of 2015‒2019 in the estuarine areas of the Kyanda, Tamitsa, Syomzha, Pyia and Chyosha rivers exposed that higher turbidity, or concentration of suspended sediments, comparing with adjacent riverine and marine water masses, is characteristic of the areas of reversing tidal currents. The sediment concentration was determined using the gravimetric method by filtering samples taken synchronously with recording the flow parameters. A wide range of turbidity fluctuations occurs depending on the tidal cycle phase, the fresh and salt water mixing, the geological and geomorphologic structure of the seashore and river channel pattern, as well as the weather conditions. The highest maximum turbidity is during the open channel season in macro-tidal estuaries, for example, 4,32 kg/m3 in the Chyosha River estuary and 2,66 kg/m3 in the Syomzha River estuary. The maximum turbidity in the Kyandy and Tamitsa rivers meso-tidal estuaries is lower by an order. Regular changes in turbidity have been traced during the tidal cycle, when the maximums of flood flow velocity and turbidity occur almost simultaneously. Increased turbidity could be facilitated by wind waves on the seashore, which entrains bottom sediments into motion, as well as by intense rainfall over the extensive tidal floodplains before their inundation. During the ice cover season synoptic factors become less important, although the tidal ice hummocking and ice contact with the bottom sediments could result in episodically higher local turbidity. As a rule, the minimum turbidity has been noted just before the beginning of the flood when fresh river water runs off. Vertical turbidity stratification in small estuaries during flood and ebb currents is practically non-existent due to intense turbulent mixing; however, under high slack water during the periods of current direction change it could appear in river pools where suspended particles settle.

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