The Tymovsky district comprises the middle and upper reaches of the River Tym and its tributaries, together with the upper tributaries of the Poronai catchment that drains southwards to Terpeniya Bay. The River Poronai is the second major river on Sakhalin Island with a catchment area of 7,990k m2. The area broadly corresponds to the Tymovskaya river group identified by Sakhalin State University (2000).
Topography and Landscape Character
The area comprises the West Sakhalin and East Sakhalin Mountains with the Tym-Poronay lowland lying in between. The West Sakhalin Mountains include the main Kamyshovy ridge with heights of 1,000 to 1,300m while the East Sakhalin Mountains comprise the Lopatinsky mountain juncture with ridges of 1000 to 1,600m in height (Sakhhydromet, 1998). The Tym-Poronay lowland is a flat waterlogged plain with a width of 5 to 15 km. The elevation of this lowland increases to the watershed between the Tym and Poronai
catchments, and is characterised by hilly relief and elevations of 80-105 m (Averina, 2004). Rivers in the area are mixed mountain–lowland rivers. Land use in the area is around 80% woodland and 4-5% marshland. Vegetation in
the mountains is dominated by spruce and fir forests with areas of birch woodland (Sakhhydromet, 1998). Poplar trees and grassland areas predominate on the floodplain of the River Tym.
Geology and Soils
The rivers within Area 2 have their headwaters in mountains composed of metamorphic rocks that are generally resistant to erosion. Soils in the
mountainous areas are predominantly brown mountainous-taiga, unpodsolised soils. (Sakhhydromet 1998). In contrast, the Tym-Poronai lowland is
composed of alluvial and marine deposits with swampy-peaty and peaty-gleyey soils (Sakhhydromet, 1998).
Climate
Tymovsky district is situated in the north of the Tym-Poronai valley and has a continental climate with large variations in temperature. The annual variation of air temperature can be up to 40°C, while daily variation can be
approximately up to 30°C. The coldest month is January when the average air temperature is -23.6°С. July is the warmest month with temperatures of 15.7 to 16.0°С (Sakhhydromet, 1998).
Precipitation
Annual precipitation within the Tymovsky district is distributed relatively evenly across the area, ranging from 710 to 760 mm. The minimum monthly
precipitation is reported to be in March (30 mm), while the greatest monthly precipitation is registered between August and October (101 mm), as is typical of a monsoon climate (Sakhhydromet, 1998).
Discharge and Flow regime
The rivers in the Tymovsky districts are mixed mountain / lowland rivers predominantly fed by melt water, which accounts for 60 % of the annual drainage volume; groundwater accounts for 25%; and rain water for 15%
(Sakhhydromet, 1998). The flow regime is therefore characterised by spring high water and summer–autumn high water periods (see Figure 3-6). The spring melt water period in late-April to June is the main stage and accounts for 50 to 70% of the annual drainage volume. The melt water period features two to three flood waves from snowmelt and abrupt fluctuation between warm and cold periods. Flooding during the melt water period on the rivers of the Tym-Poronai valley is an almost annual occurrence, caused by the
coincidence of high melt water levels, rainfall and clogging of the river channel during the period of ice break-up (Sakhhydromet, 1998). 20-25% of annual runoff occurs during the summer and autumn floods and only 5-7% of the annual runoff during the winter months (Sakhalin State University, 2000).
Figure 3-6: River Pilenga - Ado-Tymovo settlement. Drainage Hydrograph comparing periods of high and low precipitation (Source:
Sakhhydromet, 1998).
Freezing regime
The ice regime of the rivers in this area is formed under a severe and relative snowy winter. The first river ice formations normally appear in early November, but can start in mid-October or as late as mid-December. The freezing of the rivers takes 6 to 30 days. In some warm years, freezing-of the upper reach of the River Tym in the area of the Krasnaya Tym farm may occur as late as February. By late November-early December the rivers are fully frozen, this period normally lasting on average for 150-180 days. Maximum ice thickness on the rivers varies from 40 to 72 cm during warm winters and from 90 to 135 cm during harsh winters. Thawing starts to begin as soon as the air
temperature rises above 0oC and ice starts to break up during mid-late April and rivers are generally clear of ice towards late April-early May.
Figure 3-7: Average Diurnal Temperature of Air and Water - River Pilenga, Ado-Tymovo settlement (1976)
Channel Form and Adjustment
The river system of the Tymovsky area comprises more than 15,000
watercourses, the bulk of which consists of small rivers (99%) less than 10 km long. The River Tym itself is between 60 and 100 m wide and maximum depth during spring snowmelt and rainfall floods can be from 4 to 7 m. The channel width of medium-size rivers does not exceed 40m and the maximum depth is from 2 to 4 m (Sakhhydromet, 1998). In the upper mountainous reaches (e.g.
tributaries of Rivers Vosi and Pilenga) watercourses are incised within narrow V-shaped valleys and are of high velocity. Further downstream the rivers are of a more lowland character and meander over their floodplains (Averina, 2004). Channel adjustment of large and medium lowland river reaches is characterised by branching and free meandering of the river channel, together with bank erosion and meander development. Relict channels and ox-bow lakes are common and often overgrown. Floodplains are frequently swamped.
Large woody debris in the form of fallen trees influences bank erosion and planform change. Smaller rivers and brooks are characterised by sinuous channels and more gradual meander development (Averina, 2004).
Hydraulics
The velocity of rivers varies within a wide range, reflecting the different
gradients of mountainous and lowland sections of watercourses in this area. In mountainous reaches, high velocities of up to 1.5–3 m/s occur, particularly during the spring snowmelt period (Averina, 2004). During this period the stream velocity of the River Tym flowing across the associated floodplain can also reach 1.5 to 2.5 m/s (Sakhhydromet, 1998). In winter, flow velocities are typical low and the depth reaches its minimum. Data for rivers that can be found in this area is shown in Table 3.5.
Table 3-5: Water depth and flow velocity in winter for selected rivers in Area 2
Voskresenoka 0.85 0.5
Brook Kirpichny 0.8 0.3
Brook Izvestkovy 0.5 0.4
Sergeevka 0.32 0.3
Brook Tikhiy 0.2 0.5
Glubokaya 0.2 0.1
Zaprudnaya 0.41 0.4
Golubichnaya 0.2 0.2
Daldaganka 0.9 0.4
Sediment Regime
Fine sediment delivery from hill slopes is augmented by disturbance of the vegetation cover due to tree felling, forest fires and road development (Averina, 2004). The Maximum Allowable Concentration (MAC) for this particular area is 200.75 mg/l. This concentration may be exceeded during high flow events as a result of meltwater or rainfall events. For example, the average suspended solid concentration in the River Argi is 41.3 mg/l, the minimum level is 0.4 mg/l, while the maximum level recorded is 753 mg/l (Sakhhydromet, 1998).