河川の一次生産者である付着藻類は, 光合成により有機物を生産する生態系の基礎である. 従来, 付着藻類の増殖に関して, 水温や栄養塩類が対象とされ, 河川内の光の透過についてはあまり検討されて来なかった. 著者らは, 河川内の光環境と濁度が付着藻類の増殖に及ぼす影響について着目している. 本研究では, アユの漁獲数が減少傾向にある群馬県内の本川において, アユの定着した夏季に付着藻類の増殖について調査を行った. また, の4地点において, 光量子量と濁度の鉛直調査を行い, 光の水中透過率を算出した. さらに, 観測期間中に台風による洪水撹乱前後の増殖速度の違いが観測された. この結果, 付着藻類の増殖には, 河床に到達する光量が重要であり, 濁度による増殖の阻害が生じ, 攪乱後に増殖速度は増加することが確認された.

抄録全体を表示

2011年12月から2012年12月までの1年間，上流域で河川水を採取し，河川水中の放射性セシウム（¹³⁴Csと¹³⁷Cs）濃度を測定した．その結果，4月から5月の雪解けと降雨により，平水時に比べて最大で10倍高い濃度を観測した．また，雪解けによる水位上昇時の降雨と台風4号（Guchol: T1204）の降雨の影響を除くと，河川水の放射性セシウム濃度と水位・濁度との間に正の相関関係が存在した．この結果は，河川流域に沈着した放射性セシウムは，雪解けにより河川流域から河川への粒子態セシウムの流出量が増加することを示唆している．一方，雪解けによる水位上昇時と台風通過時の降雨においては，他の起源からの懸濁粒子が負荷された可能性が考えられる．

抄録全体を表示

At Mt. Tanigawa which was one of the source of Tone River basin, rainfall and snowfall of the summit were continually collected, and each nitrogen component was analyzed. The nitrogen concentration of mountain stream water which escaped from Mt. Tanigawa was also continually collected and analyzed. Then the research of river water quality was carried out for 5 times per year from mountain stream to Tone Oozeki Barrage located in most downstream edge of Gunma pref., and the transition of nitrogen concentration was observed. At the Tone Oozeki Barrage, the great bulk of water was taken as water supply resource of the Metropolitan area.
The nitrogen concentration at the summit rainfall and snowfall agreed well with that of mountain stream water. The nitrogen concentration of Tone River was over 1mg/1 from Numata city in the upstream area. At the Tone Oozeki Barrage, the nitrogen concentration was 2 to 3mg/1 through the year. These values were rather high as the resource of water supply.

抄録全体を表示

In 1783, the great eruption of Mt. Asama buried the villages of Kambara and Koyado, which lay at the northern foot of the mountain, with earth and sand and there was a major mudflow along the Agatsuma and Tone rivers further down. It was a major disaster which caused more than 1, 000 deaths.
We have clarified the nature of the flow and the deposition of its sediments from a survey on the deposits left there at the time, soil tests, paleomagnetic measurement of essential blocks and analysis of old records. We have estimated the velocity of the mudflow and its discharge at the main affected places by hydraulic calculation based on the results of the survey.
The particle size distribution of the 1783 eruption mudflow depopsits remaining along the Agatsuma and Tone Rivers roughly ranges from 0.3mm to 1.0mm for 50% particle size. The distribution hardly changes with the descent of the mudflow and closely resembles the mudflow deposition on the slope at the northern foot of Mt. Asama. The mudflow that originated on that slope thus maintained a state of equilibrium with little change in comparison as it moved down the Agatsuma and Tone Rivers.
Measurements of the paleomagnetism of huge essential blocks deposited along these rivers showed that direction of their geomagnetism generally agreed with that at the time of the 1783 eruption. It is inferred that they were deposited at more than the Curie temperature, so they might have been effused in that eruption.
Hydraulic calculation of the mudflow by the Manning formula produced a water level of 25m-100m, a flow velocity of 9m/sec-24m/sec and a discharge of 110, 000m³/sec-330, 000m³/sec for the section Mihara to Shibukawa city and it follows that the mudflow took about 80 minutes to reach Shibukawa which is about 70km from the crater. This result generally agrees with what is indicated in the old records. Water levels below Shibukawa City are difficult to estimate for many reasons, including subsequent repairs and development, and the fact that the mudflow distributed there. However, if the water level and the average flow velocity are roughly estimated at 5-10m and 3m/sec respectively from the topography of the present river channels, the flow time was generally as indicated in old cords.

抄録全体を表示

河川に生息するトビケラの幼虫に含まれる重金属元素をAASで定量し,重金属汚染環境指標生物としての同水生昆虫の評価を行う目的で,試料分解法をはじめ個体間の元素含量差,体長と元素含量との関係,季節による元素含量の変動等を検討した.確立した分析法により北関東地方数河川に生息するトビケラ幼虫中の重金属元素を定量した結果,上流域に廃鉱山を持つ渡良瀬川や桐生川では他の河川に比較して銅やマンガン等がトビケラ幼虫に多量に含まれており,同昆虫が重金属汚染環境指標生物として有用であることが明らかとなった.

抄録全体を表示

A disastrous eruption of Asama volcano, the Tenmei eruption, occurred in 1783. During the eruption, “Kanbara pyroclastic flow” buried Kanbara Village located on the northern flank of the volcano, gradually changing into mudflow facies in the downstream. Furthermore, the mudflow went down along the Agatsuma and Tone Rivers. More than 1400 people were killed by this disaster. The pyroclastic flow is distributed within a parabolic area, with a narrow angle of 30deg; from a semi-circular depression with a diameter of 700 meters, which was located 4 km north of the summit. Aerial photo interpretation and field survey revealed that many gigantic blocks of essential lithics were scattered in an area of 18.1km², and the volume of essentials was estimated to be 1.94×10⁶ m³ for blocks with diameter upper 5m and 4.30×10⁶m³ for including blocks with diameter lower 5m. The largest block was 49 meters in diameter, 10 meters high, and 9200 m³ in volume. Testpit survey clarified that the Kanbara pyroclastic flow deposit was 2.2 meters thick in average and hence amounted 4.70×10⁷ m³ in total volume. Paleomagnetic measurement of the essential lithic blocks indicated that they deposited at a temperature higher than the Curie point (about 400deg;) even in the downstream of 65km from the summit. The pipe-structure observed in the several testpits was attributed to upward steam segregation, which derived from sufficient water and incandescent essential lithic blocks in the deposit.
Considering historic pictures and documents which suggest the water overflowing during the eruption, the authors interpreted that the semi-circular depression was a swamp, “Yanai marsh”, of 1.0-2.5×10⁷ m³ in capacity.
The downflowing of Kanbara pyroclastic flow at high velocity caused an extensive erosion in the north slope of the volcano, because of much water content. The volume of the flow gradually increased since the deposit captured accidental blocks and water along the eroded area.
We propose two alternative interpretations as follows:
(1) The eruption occurred at the summit of Asama volcano and the pyroclastic flow rushed into the Yanai marsh, incorporating the water of the marsh into the flow. Consequently the Kanbara pyroclastic flow gradually changed into mudflow facies.
(2) Lateral eruption occurred at the depression (the Yanai marsh) which had been filled with water.The pyroclastic materials successively mixed with the water, which flowed out of the depression, and changed into mudflow facies.

抄録全体を表示