Changed Aquatic Environment Due to An Estuary Dam

3.1 Limited water mixing in the estuary

The water mixing regime is the most important factor in mass balancing and cycling of materials in an estuary (Knauss [1997]). After the dam building, such a mixing regime changed the circulation scheme of the estuary, consequently decreasing maximum tidal velocity by 65% from 3.5-3.8 knot to 1.5-2.2 knot during the spring tide period (Table 1).

Additionally, in situ observation showed an eminent residual landward flow at the bottom of the estuary during middle and high tide periods with low river discharge (Chung et al. [1983]; Lee et al. [1999]). Vertical stratification of water column with two opposite directions of residual currents; seaward flow at the surface and landward flow at the bottom layer were observed (Choi et al. [1995]); Kim et al. [2006]). Therefore, the dam not only physically transformed the lacustrine water into a freshwater regime but also changed the estuary to a partially-mixed type, which restricts active mixing of fresh and saline water in the estuary (Fig. 3).

Fig. 3

Vertical salinity profiles in the estuary in pre- and post-dam period; (a) in June [1982] (Chung et al. [1983]) (b) in June [1997] and May 1998 (Lee et al. [1999]). Publication permitted with the written consent from the Korea Society of Oceanography.

4.2 Hydraulic residence time (HRT) in the lacustrine water

HRT is determined by the average length of time that water remains within the boundaries of an aquatic system, and it is a key parameter in controlling the biogeochemical behavior of aquatic ecosystems (Rueda et al. [2006]). HRT in the lacustrine water is defined as following a simple equation, such as T = V/R, where, V and R are the volume of the reservoir and the river discharge, respectively (Table 1). Unlike the estuary, the lacustrine side of the dam transformed to a fully fresh-water regime, and consequently, HRT depends on the volume of the lacustrine and the river discharge. Assuming a consistent water level of the reservoir, its HRT depends totally on the river discharge of the Geum River. Therefore, the frequent heavy rainstorms and manually controlled low water-level of the reservoir resulted prolonging the HRT by a factor of five to 11.6 days during the wet seasons; and HRT was prolonged even more greatly by a factor of 10 to 23.78 days during the dry season after the damming.

Table 1

Summary of environmental consequences due to the Geum-River Estuary Dam System

4.3 Estuarine HRT

Unlike the lacustrine HRT, estuarine HRT (T_r) is defined as the time required to replace all of the water with the tidal volume and river discharge (Table 1). The numerators, V, P, and T, stand for the low-tide volume, the tidal prism, and the tidal period of the estuary, respectively. This entire component of numerators represents the time-averaged sum of all of the water volume (capacity) in the estuary. The denominators, b and R, symbolize the return flow factor (set to 0.5 (USEPA [1985])) and the river discharge, respectively. These two components correspond to the actual time-averaged sum of all of the water volume (tidal flow and river discharge) in the estuary. Therefore, T_r (HRT) means the time required to replace all of the estuarine water with the tidal volume and river discharge. The actual difference in the HRT calculations between the estuary and the reservoir is their water composition, as there is only freshwater in the reservoir, and there is seawater plus freshwater in the estuary.

Since the dam was built, the maximum velocity of the tidal current has decreased, as did the tidal prism of the estuary (Table 1; also refer Kim et al. [2006]). The tidal prism, expressed as P, was reduced in both the numerators and denominators, but slightly less in the denominator due to the effects of the return factor, R. The estuarine HRT was not considerably different from the pre-dam situation, only prolonged by 22.1%. The HRTs increased on both the lacustrine and estuarine sides but to a much greater extent in the lacustrine water. Therefore, the physical consequences after the dam building, such as the extended HRT, the limited tidal current, and the freshwater regime, were much more noticeable in the reservoir than in the estuary.

5.1 Elevated nutrients concentrations

Since the dam was built, due to limited tidal currents in the estuary, the high concentrations of SS in the estuarine and lacustrine water subsided down to 41.7 mg L^-1 and 21.9 mg L^-1, respectively (Fig. 4). In addition, much more significant increase of PO₄^3-, NO₃^-, and NH₄⁺: have been found in lacustrine water (130%, 98%, and 193%) than estuarine water (200%, 168%, and 767%) (Table 1).

The lacustrine water showed less turbidity, containing half of the SS concentration of the estuarine water. Consequently, there is much less photo-inhibition of APP in the reservoir than in the estuary. Since the dam was built, the water quality on both sides of the dam have experienced elevated nutrient levels and water clarity, though the lacustrine water suffered from deteriorated water quality more seriously due to greatly escalated Chl-a and nutrients levels than the estuarine water (Jeong et al. [2010]). These concentrations were so high that this lacustrine water quality represented a hypertrophic state according to the OECD Water Quality Standards.

Fig. 4

Long-term variations of suspended solids in the Geum-River Estuary Dam System. Dotted line indicates the dam building in [1994] (cited from Jeong et al. [2010]).

5.2 A bay-like character estuary

During the wet season, the increased river in-flow due to high precipitation predominant other physico-chemical processes in the estuary. However, the lacustrine effluents flow into the estuary sporadically through the sluice gate during the dry seasons. Due to these strong seasonality, the water quality of the estuary strongly depends on the fluvial input manifesting a conservative character for NO₃^-, NO₂^- and SiO₂^- throughout the year, though there is an additive pattern for PO₄^3- and NH₄⁺ during the wet seasons (Yang et al. [1999]). Additional processes affecting nutrients are desorption processes from SS and the destruction of organic materials, mainly from phytoplankton biomass contained in the lacustrine effluents. During the dry seasons, a low salinity area existed, ranging from 515 psu, within less than 1 km away from the sluice gate of the dam. Therefore, unlike the pre-dam period, the estuary maintained a bay-like character, especially during the dry seasons in the post-dam period.

6.1 Elevated autochthonous primary production (APP) in the reservoir

Based on another hyper-trophic estuary dam in Korea, the Nakdong Reservoir, autochthonous primary preproduction (APP) is usually under the strong influence of phytoplankton biomass, irradiance and nutrient availability (Jeong & Cho [2004]). Because phytoplankton biomass and nutrients were sufficient for APP, it was concluded that the lacustrine APP linearly correlated with total irradiance. Accordingly, the APP exceeded the production of allochthonous origin (Choi [1994]), and accounted for more than 50% of the organic load (Shin et al. [1996]).

We observed an almost identical situation in the GREDS. Over the last two decades, the Chl-a concentration has increased by more than one order of magnitude, and other APP-related parameters, such as DO, pH, and COD, have increased by a factor of two (Jeong et al. [2010]). After the dam building, nutrients and DIN/DIP concentrations increased by several folds, and their seasonal fluctuations became much greater than before. Therefore, APP has increased much more in the lacustrine water than in the estuarine water since the dam building.

6.2 Harmful algal bloom (HAB) in the reservoir

Before the dam building, the dominant phytoplankton species in the lacustrine water were benthic brackish diatoms, such as Navicula, Skeletonema, and Thalassiosira (Shim & Yang [1982]). At that time, strong tidal currents and murky water inhibited primary production, so that a very low autochthonous algal biomass and very low phytoplankton species diversity were present. However, after the dam building, the lacustrine side changed to a freshwater regime, and planktonic fresh water diatoms such as Stephanodiscus appeared to be dominant in winter, and the blue-green algae Microcystis became dominant in summer (Shin & Cho [2000], also refer Table 1).

Microcystis, a well-known HAB species, prefer high temperature and phosphate concentration with lentic water, instead, diatoms require optimum soluble reactive nutrient ration of 1 (Si:N) for active growth (Humborg et al. [1997]). Based on this suggested that frequency of diatom blooms over the last decades has decreased and dinoflagellates have become more important, especially during summer, due to the dam building. Still this hypothesis is hard to prove because of constantly changing nutrient concentration and other conditions involved in the GREDS, such as northern jetty, etc.

6.3 Phytoplankton communities in the estuary

Since the dam was built, autochthonous blooms of Mesodiniumrubrum with concentrations of 6,950-8,650 cells ml^-1 have repeatedly appeared in the estuary (Yih et al. [2000]). This euryhaline species is known to compete with other coastal plankton, with a wider tolerance to salinity and less dynamic conditions in the post-dam period. Addition to this species, other dominant phytoplankton were euryhaline coastal diatoms such as Skeletonema costatum, Asterionella glacialis and Thalassiosira nordenskiodii, and freshwater algae such as Anabaena sp., Oscilatoria sp., Stephanodiscus hantzscii, Pediastrum boryanum and Cryptomonads, which probably simply passively flowed into the estuary with lacustrine effluent (Table 1).

7.1 Increased siltation rate in the reservoir

Siltation is a major problem in reservoirs and dams because it reduces water storage capacity, threatens their roles as water supplies, shortens their lifetime, and finally increases their maintenance cost. Every year, approximately 880 million tons of agricultural soil is deposited in American reservoirs (Biswas [1996]). Thus, siltation has posed a major problem for the economic life of dams around the world.

The water basin of the Geum River is one of the most intensively farmed agricultural areas in Korea, and conventional tillage practices in this watershed often result in excessive soil erosion, leading to increased turbidity in the Geum River flow, especially during the wet season, including the Asian monsoon period.

Recent soundings of the lacustrine bottom topography in this reservoir recorded approximately 3-7 m thick sediment that has accumulated at the bottom of the reservoir after the dam building (Lee et al. [2005]). This previous study also found mud layers at the surface, an alternating silty-sand and mud layer in the middle, and a sand layer at the bottom, to a depth of 7 m. Before the dam building, there were only sand layers because of the dynamic condition of the estuary; all of the mud and silty-sand layers accumulated afterwards. For instance, based on a simple calculation of an average sedimentation rate of 500 cm 20 years^-1, and assuming this sedimentation rate maintains in the future, we obtained a sedimentation rate of 25 cm year^-1. Thus, as the average depth of the reservoir is only 3 m, we calculate that it will be filled by siltation within 15 years. Even though this kind of calculation may be rough and possibly incorrect in some aspect, the calculated rate is high enough to attract our attention.

Before the dam building, 5.6×10⁶ tons year^-1 of sediment were transported to the estuary by the Geum River (Chough & Kim [1981]). However, this reduced to 2.94×10⁶ tons year-1 after the dam building (Kim et al. [2006]). Thus, we could easily estimate an additional 1.83×106 m³ year^-1 of sediment trapped in the reservoir. A previous study reported 0.56×10⁶ m³ year^-1 of sedimentation in the reservoir before the dam building (KRC [1996]), so annual sedimentation increased by 209% after the dam building (Table 1). Therefore, the GREDS traps a significant portion of the river-transported sediment that passes through this system. Due to such a rapid accrual of sediment, the reservoir will not be able to be used for its original purposes in the next decade (Lee et al. [2005]). Therefore, this serious siltation problem is threatening the original role of the GREDS.

7.2 Changed sedimentary condition in the estuary

Before dam building, the estuary showed characteristics of a typical macro-tidal estuary, with its tidal excursion reaching up to 60 km upstream from the mouth of the estuary (Fig. 1). At that time, an enormous volume (approximately 1.0×10⁹ m³ day^-1) of tidal floods and ebb currents swept over the entire channel of the estuary, and the HRT of the estuary was estimated to be 1.90 and 1.94 days for the wet and dry seasons, respectively (Table 1). However, the maximum tidal current velocity decreased by 65.5%, changing the estuary to a partially mixed type characterized by vertical stratification during the dry seasons after the dam building (Fig. 3). Under this calm condition, abundant particulate matter began to settle down to the bottom, and SS concentration in the water column decreased by 73.0% from 154.3 mg L^-1 to 41.7 mg L^-1 on average (Fig. 4). Thus, the sedimentation rate increased by 91%, from 3.5×10⁶ m³ year^-1 to 6.7×10⁶ m³ year^-1 (Table 1), with deposition as high as 32-66 cm year-1 in the estuary after the dam building (Kim et al. [2006]).

In addition to this increased sedimentation rate, the composition of sediment also changed. Because the reservoir efficiently traps relatively large grain particles before reaching the estuary (Lee et al. [2005]) and because of the less dynamic conditions due to the weak tidal action in the post-dam period, most bottom sediments are covered by mud deposits in the estuary (Kim et al. [2006]).

For the GREDS, the sediment in both the estuary and reservoir are accumulating so rapidly that the reservoir will be filled up in the next decade, and the estuary will no longer function as a harbor. Thus, due to such an extremely high sedimentation rate, the GREDS will not be usable for its original purposes in the next decade.

7.3 Accumulation of organic-rich sediment in the reservoir

The inflow of a large amount of total phosphorous (TP) to reservoirs results mainly from erosion of soil particles from steep slopes and disturbed ground during floods and monsoons (An & Park [2002]). Thus, TP is commonly associated with soils, while only a small percentage of the TP in freshwater is in the soluble forms of PO₄^3- and its polymers. In the watershed, PO₄^3- is readily immobilized in the soil, and even that from fertilizers soon becomes unavailable for plankton growth. Thus, in contrast to other nutrients, PO₄^3- does not move easily with ground water to recharge surface streams (Horne & Goldman [1994]). More than 90% of TP eroded from continental rocks is carried to the sea through estuaries in an inert form, and total SS positively correlated with TP in Missouri reservoirs (Hoyer & Jones [1983]; Jones & Knowlton [1993]). Consequently, phosphorous, especially its particulate fraction, may have settled under tranquil hydraulic conditions, like those present the post-dam period, together with inorganic particulate materials, such as SS.

As improved water clarity with enhanced light penetration provides better condition for primary production, acceleration of APP was observed by previous investigators soon after impoundment of oxbow reservoirs (Cullum et al. [2006]). They reported that reduced sediment-laden run-off should reduce SS concentrations in the receiving water of reservoirs. Recently, frequent autochthonous algal blooms have produced abundant organic materials in lacustrine water, and significant parts of those settled down to the bottom of the reservoir (Shin & Cho [2000]). Consequently, the surface sediment of the reservoir is covered mostly by a thick mud layer containing rich organic content. According to the preceding investigators, the chemical properties of the sediment in the reservoir showed that organic carbon, TP, organic P, and ignition loss were 0.75-1.66%, 680-1,237 mg kg^-1, 200-705 mg kg^-1, and 3.0-10.9%, respectively (Kim and Lim [1997]; Hwang et al. [2001]; Chung and Hwang [2002]; Lee et al. [2005]). Moreover, they estimated the maximum releasing rate from sediment as 16.6 mg TP m^-2 day^-1, which would correspond to 35% of the mean TP concentration in the lacustrine water. Therefore, silty sediment with rich organic content exists at the surface of the lacustrine bottom sediment, due to the active APP in the reservoir.

7.4 Organic rich sediment and turbidity maximum zone (TMZ) in the estuary

Turbidity maximum zones (TMZ) exist in many estuaries of the world, being important with respect to the sedimentary processes of fine materials in estuaries (Allen et al. [1974]; Doxaran et al. [2009]) and to its evolution pattern in temporal and spatial contexts (Uncles et al. [2002]). For the macro-tidal estuarine environment before dam building, a TMZ existed in the middle of the estuary approximately 15-20 km from the mouth (Lee & Kim [1987]). As schematically illustrated (Fig. 5a; also refer Kim et al. [2006]), the TMZ was generated during the period of the spring tide, and it was concentrated at the middle of the estuary during the neap tide, while it was flushed out from the estuary during the high river discharge period. Therefore, the tidal current and fluvial regime was two key parameters causing the existence of the TMZ. In the central core of the TMZ, an SS concentration up to 1,000 mg L^-1 was reported at the bottom layer during the spring tides (Lee & Kim [1987]), which moved back and forth with tidal excursions along the estuary and was flushed out of the estuary with high river discharge.

Fig. 5

Schematized diagrams of seasonal sedimentation processes in the Geum River Estuary Dam System under (a) pre- and (b) post-dam conditions. The open arrow indicates residual flow and the filled net sediment transport. The magnitude of net flow and sediment flux is expressed implicitly by the length of the arrows. Erosion occurs during high river runoff in rainy summer and accumulation of sediment in dry winter at the adjacent location to the dam (Kim et al. [2006]). Publication permitted with the written consent from the Elsevier Publisher and “Estuarine, Coastal and Shelf Science”.

Unlike in the pre-dam period, the current tidal excursion is limited to only 15 km from the mouth of the estuary by the dam, and the maximum velocity of the tidal current has decreased by 65%, with SS dwindling down to one fifth of its pre-dam levels. The existence of a high SS concentration in the estuary would likely be the result of strong resuspension of sediment and/or intrusion of seawater carrying suspended sediment from coastal waters (Uncles et al. [2002]). In the GREDS, the strong tidal current has diminished, though fine particulate materials with rich organic content continue to flow into the estuary. The fine particles in the estuary originated from accelerated APP in the reservoir. Therefore, the sedimentary scheme of the estuary has changed, as illustrated in Fig. 5b (Kim et al. [2006]).

Even after dam building, fine particles and dissolved organic materials from the reservoir continue to flow into the estuary, produce flocculation in contact with saline water, and settle down near the sluice gate of the dam, where there is organic rich sediment. As a result, a high sedimentation rate up to 32 cm year^-1 and rich organic content was observed in the estuary (Kim et al. [2006]).

The most significant characteristic of an estuary is the mixing of freshwater and seawater, which are different in their physical and chemical natures (Aston [1978]). The inflow of freshwater into the dense seawater in an estuary produces a vertical salinity gradient, and this vertical stratification controls the transport of water and SS within an estuary (Allen et al. [1980]). Because the Geum River Estuary showed a wide tidal range, the fluvial in-flow and tidal conditions seem to be responsible for the classification of the estuary. As the vertical salinity profiles showed particularly high stratification profiles ranging from 5-20 psu (Fig. 3), the estuarine mixing type with two-layer flow has become predominant, suggesting the influence of freshwater at the surface layer and saline water at the bottom. During the flood tide, landward transport prevailed in the estuary; in addition to this type of transport, secondary landward movement accompanied the returning flow with highly concentrated bottom sediment and coagulations of dissolved river-transported materials (Allen et al. [1980]). After dam building, the altered sedimentary scheme in the estuary, characterized by less dynamic conditions and a stratified water column, induced the high accumulation rate of sediment with rich organic contents.

8.1 Trapping of river-transported materials in the reservoir

The Geum River Estuary was a typical estuary with dynamic physical conditions with macro-tidal amplitude of 7.5 m, a high concentration of suspended solids of frequently more than 150 mg L^-1 (Lee & Yang [1997]), and low primary productivity due to high turbidity (Shim & Yang [1982]) before the dam building. At that time, extremely murky water from high SS content was an important inhibiting factor limiting autochthonous primary preproduction (APP), even though there were sufficient dissolved nutrients in water. Therefore, materials in the fluvial water, such as nutrients, pass through the estuary intact without being fully depleted by phytoplankton consumption. Because of this, the estuary was functioning almost as a simple passage for river transportation of materials to coastal waters.

Since the dam was built in 1994, two-thirds of the total estuary area have transformed from a brackish to a fresh-water regime, rapidly replacing salt water. Consequently, most of the SS in the lacustrine water settled down to the bottom, and eventually the water transparency improved (Fig. 4b). Correspondingly, calm water with high transparency, high nutrient concentrations and elongated HRT in the lacustrine water has provided an ideal condition for the growth of phytoplankton. Consequently, plentiful nutrients in clam water probably have a much greater opportunity of being consumed by APP. Finally, the lacustrine water after dam building transformed to a highly eutrophic water body with frequent autochthonous algal blooms and elevated nutrient concentrations (Jeong et al. [2010]). Eventually, the lacustrine water became extremely productive, and the active conversion of nutrients to organic materials occurred. During the last two decades, significant amounts of NH₄⁺ and PO₄^3- have been removed from the lacustrine water, while a corresponding quantity of Chl-a and organic materials have been generated in the lacustrine water. Therefore, this active transformation of dissolved nutrients into particulate organic materials in the reservoir has converted the role of the Geum River Estuary from a simple passage to an effective trap of river-transported materials.

8.2 Ecological consequences due to the excessive trapping of river-transported fine particles in the estuary: a potential hazard to coastal water

The estuary used to behave like a passage for river-transported dissolved nutrients because of its dynamic conditions with aggravated suspended solids and poor clarity. Such a condition is inhibitory to primary production, even with plenty of nutrients.

Estuaries, in nature, represent a pathway for materials from land to coastal waters and function as a filtering system. However, the artificial structures like the estuary dams disrupt their normal functions as estuaries.

After the dam building, only one-third of the total estuarine area has remained as an estuary. Strong tidal currents no longer exist, and SS has decreased to one fifth of its previous concentrations. Therefore, photo-inhibition of primary production has disappeared. In addition, the sedimentary scheme has changed. The TMZ has disappeared, and the stratification of the water column has produced a two-layer system with freshwater at the surface and saline water at the bottom. Moreover, an evident landward movement of saline water through the bottom layer has proved to be a favorable condition for the sedimentation of fine particles, especially coagulated materials formed from freshwater contacting saline water. Through this process, rich organic sediments have accumulated at very near to the sluice gate of the lacustrine region. Currently, even with only one-third of its original area, the estuary traps sediment and organic matter more efficiently than before. Therefore, the estuary is no longer functioning as a passage for river-transported materials, but rather like an efficient trap (Table 2). One of the active removal process of materials due to the damming, such as “missing silicate” hypothesis probably explain one of the reasons for the declines of coastal fisheries in the Black Sea (Humborg et al. [1997]).

Table 2

Summary of ecological consequences due to the Geum-River Estuary Dam System