狭は我が国最古のダム形式の農業用のため池であり、多くの文献によってその築造の歴史や改修の記録が残されている。現在、大阪府ではこの池を新たに洪水調節機能をもつ治水ダムとして活用する事業に取り組んでいる。今回の事業を進めるなかで、これまで数々の貴重な土木遺構が発掘され、狭の築造の歴史と過去の土木工事の様子が我々の目前に姿を現してきた。古墳時代から昭和の時代までの狭の用水による農業生産によって生活を支えられてきた多くの農民や当時の指導者達のこの池の築造と改修への努力と貢献が遺構や文献から明らかとなっている。
以下の文ではこれら歴史的ダムの保全の事業の一部を紹介し、また改修の歴史を刻む堤体断面や数々の土木遺構を通じて、それぞれの時代での取り組みの有様を明らかにするとともに、今回発掘された貴重な歴史的土木遺構を将来へ継承するため、建設を計画している狭ダム資料館 (仮称) の概要を紹介するものである。

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　本研究の目的は衛星センサによる汽水化直後の湖（鳥取県）の水質モニターの可能性を検討することである．筆者らは2012年における3時期の現地調査に基づき，湖水の分光反射率特性と水質（Chla：クロロフィルa）との関係について議論した．その結果，以下のような事項が明らかになった．まず最初に，湖の湖水は典型的な富栄養化水域の分光反射率特性を示した．次に，RSによるChla推定に最適なモデルは「672nm（±約5nm）と704nm（±約5nm）の反射率比」であった．現行の衛星（WorldView-2）の659nmと724nmの反射率比とChlaは有意な相関は得られなかった．以上のことから，現行の衛星で単純な反射率モデルを使って湖のChla推定を行うことは非常に難しいが，近い将来打ち上げられる予定であるHSセンサを使えば，湖のChla推定が可能であることが示唆された．

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鳥取県湖を対象に, 湖底付近が最も嫌気的な状態となる夏季において底質調査を実施し, 底質中の重金属 (Fe, Mn, Cu, Pb, Zn) および硫化物の濃度分布特性にっいて検討した.その結果, 各重金属ともに最深部を含む池北西部で高濃度を示すなど, 類似した分布であった.硫化物濃度は最深部周辺で局所的に高濃度を示すものの, 全体的に濃度は湖沼における全国平均値を大きく下回り, ヤマトシジミの硫化物耐性の観点からも極めて低い値であった.また, すべての底質調査項目を用いた主成分分析の結果, 底質の汚濁の程度と還元状態の強弱を表す指標が導かれた.さらに, クラスター分析により湖の底質は5っに分類することができた.

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狭ダム事業は、日本最古のダム形式の溜池である狭を改修し、洪水調節機能をもつ治水ダムとするものである。事業にあたっては、この池の歴史的価値に鑑み大阪狭山市の協力のもとに種々の調査に取り組み、これまでに池の最初の築造時期が西暦600年前後であることなどが判明している。
本稿ではこの初期の堤体が造られた時代的背景や築堤工法等を検討し、盛土に際して、葉のついた木の枝を層状に敷き作業の効率化を図るなど当時の高度な土木技術が用いられたことなどを紹介するものである。

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鳥取県湖を対象に密度成層の形成過程と破壊過程について鉛直1次元拡散モデルにより検討した. 鉛直渦動拡散係数は局所リチャードソン数の関数としてモデル化し, それに含まれるパラメータは遺伝的アルゴリズムを用いて水域に最適な値として決定した. その結果, 本モデルは実測値を良好に再現できた. 数値実験の結果, 風速5.0m/s以下で水温日成層が形成され, 風速により密度界面の位置が1.5～3.0mの範囲で変動することを示した.また, 成層破壊に要する吹送時間は風速の指数関数で与えられた. さらに, 水深約6.0mの最深部において夏季に3.0m以深で貧酸素化・無酸素化になる要因を, 数値実験の結果を踏まえて風速, 密度界面, 補償深度と関連付けて考察した.

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To examine the behavior of DO from the viewpoint of formation and the break of density stratification, we continuously measured the water quality and micrometeorology of Lake Koyama at its deepest point of about 6 m from June to November in 2003. DO and EC in the surface and bottom layers, the vertical profile of the water temperature, the wind velocity and insolation were observed. As a result, DO in the bottom layer was lacking due to the formation of the thermal stratification in summer, and due to the saline stratification in a non-irrigation period after a large amount of sea water had flowed into Lake Koyama. The results also revealed a negative correlation between DO in the bottom layer and Wedderburn number W. Further analysis of the relationship between them indicated that anoxic water was generated due to the repression of the DO supply below surface layer under the stratification conditions of W>4.5, and that the anoxic state in the bottom layer could be eliminated by the generation of vertical circulation when the wind velocity of W<4.5, more than 4.0 m/s in summer and more than 10.0 m/s in the non-irrigation period, operated on the water area.

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　本研究の目的は，鳥取県中央部に位置する汽水湖の東郷池において，非接触でChlaを推定する手法を検証することである．この検証のために2013年5月から2015年7月までに取得された42個（4時期）の実測分光反射率／Chlaデータセットが取得された．その結果，「湖モデル」（672 nmと704 nmの2波長比）では40μgL^-1以上の高濃度部で大きな誤差が生じた．この誤差を抑制するために，水中の生物光学的な理論に基いた新しい3波長モデルを取り入れて，この湖に適した計算式や設定波長について検討した．最終的には，適切に波長選択された3波長（650 nm，703 nm，740 nm）を使った場合，Chla推定誤差は「湖法」と比較して約8%の大幅な精度向上があった．

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The formation of water blooms with blue-green algae Microcystis spp. has been observed in Lake Koyamaike (Tottori Prefecture) every year, and the changes of the water quality for the worse is of concern. An algal growth potential assay using Microcystis aeruginosa isolated from the lake and water quality examination of the lake and four inflowing rivers were carried out in order to determine the mechanism of the heavy algal growth. Both examinations were continued every month from April 1995 until August 1998.
The substance limiting the algal growth was estimated to be chelate compounds. This proved true throughout the year except during the summer. The chelate compounds were found not only in the lake water but also inflowing river water in June. This was attributed to be basal fertilizer which contains molasses used in paddy fields. The algal growth under the elimination of the effects of nitrogen and phosphorus showed good correlation with the copper complexing capacity.

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Lake Koyama is a low salinity lake that is connected to the Japan Sea via Koyama River. The seawater inflow into the lake is controlled by a sluice gate set up in the river. In summer there is no seawater inflow because the lake water is used for irrigation. On the other hand, in winter, to promote the use of the lake as a fishery, there is an artificial seawater inflow around November. This paper details observations performed to see the change in water quality when seawater flowed into the lake in the winter of 2001. The following has been understood as a result.
The seawater quality was found to have already deteriorated around the mouth of Koyama River. When this water flowed into the river, it became worse, because nutrients emerged from the bottom mud and sewage was carried into the river. Moreover, when the seawater entered the lake it concentrated in the vicinity of the deepest part. As a result of this, the dissolved oxygen of the lower layer disappeared for a few days after the inflow and this made the water quality worse. This situation continued for about 20 days until destratification by a strong wind occurred.

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Using a 150-cm lake bed core sample, the paleoenvironmental change during the past ~1,300 years in Lake Koyama-ike, Tottori Prefecture, western Japan, was reconstructed based on diatom fossil assemblages, electric conductivity, and radiocarbon ages. Consequently, the following points were clarified:
1) During the past 1,300 years, the surface water of Lake Koyama-ike has been a freshwater environment—freshwater pond or lake—because freshwater diatom species were dominant in the entire core. In contrast, due to salinity stratification, a brackish-to-marine environment has remained immediately above the lake bed.
2) The diatom fossil assemblages indicated the decrease in water salinity and depth after ~1,000 cal BP. Because a similar transition has been previously reported in studies conducted at other sites in Lake Koyama-ike, this environmental change is a common phenomenon in this lake.
3) A decrease in water salinity around 1,000 cal BP indicated that the channel connecting the Japan Sea and Lake Koyama-ike was filled by the blown sand. The shallowing of the lake around the same period was probably caused due to rapid deposition of riverine sediments and relative sea-level fall, which presumably induced the sand to actively blow in the coastal area.

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狭は古事記や日本書紀にもその築造が記載されている日本最古のダム形式のため池であり、様々な文献に狭の修改築の歴史が記述されている。大阪府の調査の結果、堤体には幾多にわたる過去の修改築の跡が層になって残されていることが明らかになり、また古代の堤には木の葉が10cm～15cmのピッチで水平に敷かれた古代の築堤工法 (敷葉工法と呼んでいる) も見られることがわかった。
大阪府では狭のダム化工事の中で、旧取水施設の撤去のため堤体を一部全断面開削する機会に、この歴史的にも土木工学上も貴重な堤体を一部全断面で保存展示することとした。
本稿は、堤体の保存を実施するための調査検討や現在までの堤体保存工事の実施状況について報告を行うものである。

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Lake Koyama-ike, located in Tottori Prefecture, western Japan, is a coastal brackish lake facing the Japan Sea. Since 1983, this lake has been characterized by artificial fluctuation of its lacustrine water quality. We collected lake bed surface sediments from 25 sites of the lake on July 10, 2010, corresponding to a period when the water salinity had been controlled to be in the range 150–1,000 mg/l. The diatom assemblages in the samples comprised a total 137 diatom species belonging to 38 genera. Among them, the major 11 species belonging to 8 genera, i.e., Aulacoseira granulata, A. ambigua, Cyclostephanos dubius, Cyclotella atomus complex (C. atomus var. atomus and C. atomus var. gracilis), C. meneghiniana, Cyclotella sp., Diploneis sp., Fallacia florinae, Lindavia praetermissa, Thalassiocyclus lucens, and Thalassiosira lacustris, occupied more than 70% of all samples. Results of the clustering analysis indicate that similar diatom assemblages were found at most sampling sites and their distribution pattern was unclear. We interpreted the axes of the principal component analysis as water salinity (axis 1) and distance from the lake's shore (axis 2), respectively. Totally, seven of the predominant diatom species, except for L. praetermissa, F. florinae, and two unidentified species (Cyclotella sp. and Diploneis sp.), were common in the periphytic diatom flora near the lake's shore.

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Seawater backflow from the Sea of Japan is controlled by operating the flood gate on the Koyama River between the Sea of Japan and Lake Koyama-ike, a brackish lake in the Sanin region. An attempt was made to evaluate the salinity flux at the mouth of Lake Koyama-ike quantitatively, focusing on the effect of salinity on the ecosystem of the lake. The salinity and flow rates were monitored using electrical conductivity loggers and Doppler flow meter readings obtained every 10 min at various depths from August 8, 2021 to October 21, 2021. Based on the observed results, the salinity flux at the mouth of Lake Koyama-ike was estimated to be 0.09–5.82 kg/m2/s into the lake. When the salinity flux was high, the water level difference between the lake and the downstream site of the flood gate was large, the opening of the flood gate was large, and the salinity concentration was high at the downstream site of the flood gate. However, the results suggest that when the salinity flux is small, the flood gate opening is small, salinity concentration at the downstream site of the flood gate is low, and backflow time tends to be short.

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The field observation conducted in the summer of 2000 at Lake Koyama showed that there were some types of internal waves. In this study we simulated those data using ELCOM which is a three-dimensional hydrodynamics model developed by CWR, the University of Western Australia. The results of simulations showed that there were three types of internal waves. The first one is an internal seiche generated after the stop of a sea breeze, the second one is an internal seiche after the stop of a land breeze and the final one is an internal Kelvin wave after the stop of a sea breeze. It is also cleared that in the water body of the lake these waves interacts each other and this causes the complicated distribution of water temperature and flow structures under the condition of land and sea breeze.

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In Koyama lake, phytoplankton blooms appear in summer season. This phenomenon might be influenced by the water quality. Therefore in this paper, in order to investigate the mechanism of phytoplankton bloom, the water qualities.in Koyama lake such as pH, Do, turbidity, nitrogen and phosphorous were observed from 1991 to 1993. And characteristics of these water qualities were also discussed.

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