玄手川水路底改修に伴う水生植物の回復 Restoration of waterweeds in a remodeled channel
The river Gente is a 3,000m-long river used as a drainage channel and flows north down the northernmost part of the Shogawa alluvial fan. The stream is fairly clean thanks to an abundant supply of seepage water welling up out of the underground and abounds in aquatic plants and animals such as Tomiyo and Sparganium, the latter of which is already listed as a semi-endangered species in the red data book. The revetments of the river were once renovated using dry concrete blocks (1970-1972) but the soil riverbed was left untouched, as was usual with the renovation of a drainage channel in those days. Thus waterweeds still grew wild, incurring both a reduction in the drainage performance of the channel and an increase in its M & O cost. This predicament prompted a further renovation of the channel. This time 80% of the riverbed was covered alternately with flat concrete blocks and gravel-filled blocks (1996-2000). This construction design was adopted for the dual purpose of facilitating waterweed control and minimizing damage to aquatic life. I surveyed the effects this riverbed renovation has produced on the ecosystem of this river. In this research, for an indication of the restoration of the aquatic ecosystem I focused on the waterweeds, in which fish nest. 70% of the waterweeds had originally been accounted for by Sparganium, and the rest mainly by five other kinds of waterweeds such as Ranunculus. I found the original ratios of these plants were restoring in about 3 years. And the coverage rate of waterweeds over the riverbed, which had been 67% before the riverbed renovation, was restored in about 4 years. This shows that it takes at most four years after a riverbed renovation for the original state of the waterweeds to be restored on both counts.
富山県立大学紀要 11, 65-75, 2001-03-30