荒川中流部扇状地河道における近年の砂礫堆の発達―自然的・人為的環境変化に対する河川地形システムの応答―

荒川中流部の扇状地区間で，砂利採取などを発端とする河床低下が1970年以降にほぼ終了した．それに合わせるように扇央部では，砂礫堆が形成した．その堆積物を研究対象として，層相・層序を詳しく観察し，9層の洪水堆積物を識別した．一方，この期間に，上流約15kmの地点で観測された年最大流量記録から，大小13回の洪水発生を認め，洪水堆積物を対比した．堆積物中に含まれていたインクペンの製造年を，一つの有力な年代指標として用いた．それによって1970～1993年までの堆積物と洪水イベントとの対応が認められた．その堆積物は洪水時に供給され，砂礫堆上流側の砂利採取に起因する河床低下の影響と扇頂部まで遡及したことで明戸サイフォンが露出し，それが一種の造瀑層となったことで生産された土砂が供給された．その土砂供給地点の明戸サイフォン直下では，深さ約5mにも及ぶ洗掘が引き起こされた．上流側には明戸サイフォンの露出をきっかけにそれを局地的侵食基準面として約2m程度の河床低下をもたらした．上流側での河床低下は六堰頭首工の堤体の一部や，その付近の護岸の破壊につながり，下流側では洗掘地点に続いて長さ約2km程度の穿入河道が形成，固定された．これらから，いわば人為の影響によって供給された砂礫が，観察地点におけるいくつかの洪水堆積物に含まれていることが確認された．また，河道の安定化にともなう砂礫堆表面での植生定着・成長が洪水による土砂の堆積傾向が強化され，伏流水の増大・河流水の減少を促したことが推察された．これを河川地形システムが自然的・人為的インパクトに応答した過程の一端としてシステム論的に解釈すると，大小の洪水が発生する中で，砂利採取や人工構造物の設置・破壊・改修という人為作用が，長短さまざまな時間スケールで河道付近の微地形形成に関与していることが確認できた．

Gravel-bars were formed at a point in the alluvial fan of mid-Arakawa, where river-bed degradation continued in some reaches even after the formal finish of gravel mining in 1970. I intend to reconstruct the processes of gravel-bar development on the basis of detailed stratigraphic observation of gravelly deposits and the reference of official discharge records. I distinguished 9 gravel beds which seem to correspond to respective flood events while 13 large and small floods were recorded since 1970 at a river gauge about 15km upstream. Each gravel bed was correlated with single flood event based mainly on the magnitude and time sequence of floods. A pen included in one of the gravel beds provided a useful key of correlation because its date of manufacture was known. The correlation enabled the interpretation of the effects of man-made structures on fluvial processes. For example, the exposure of the Aketo-siphon, which was buried in bedrock in 1939, exposed around 1970 as a consequence of retrogressive down cutting induced by gravel mining and affected the river bed in the following two ways. In the downstream reach, very localized incision at the foot of the siphon produced gravelly sediments from the bedrock exposed in the incised reach. In the upstream reach, the exposed siphon acted as a local base of erosion and induced about 2m lowering of riverbed which made the seawall near or part of the dam body of the Rokuseki-headworks unstable. The unstabilized structures were destroyed by the floods in 1981 and 1986 and the fragments were transported downstream. Both gravelly sediments produced in the incised reach lower than the Aketo-Siphon and the concrete fragments provided from the Rokuseki section were included in the gravel bed sediment at the observation point. Increase of depositional trend since around 1980 in the middle fan reaches lower than the observation point promoted the increase of underflow and resulted in the decrease of surface flow. The situation affected the growth of vegetation on the gravel bars. The recent development of landforms in and around the river channel of the mid-Arakawa is thus interpreted as a response of fluvial system to natural and man-induced impacts.