1983 Volume 38 Issue 1 Pages 11-19
Dewa Hills in Yamagata Prefecture is composed of three mountainous regions: the Chokai Volcano, Dewa Hill (in the narrow meaning) and the Gassan Volcano. The mountains are dissected by several rivers, such as the Nikko-gawa R., the Aisawa-gawa R. and the Tachiyazawa-gawa R. These are representative of the each region mentioned above in the order. The present investigations have been conducted with the aim at elucidating the hydrological and geochemical features of the rivers by comparing each other. The results are summarized as follows: 1. The Nikko-gawa R. forms parallel basin composed of two featherlike basins. The drainage basins of the Aisawa-gawa R. and the Tachiyazawa-gawa R. are radial and featherlike respectively. Therefore, the average basin width of the Tachiyazawa-gawa R. is minimum. 2. In regard to the development of the tributaries, the first and second laws of Horton are applied to the above mentioned rivers. The bifurcation ratio of the Nikko-gawa R. is nearly equal to that of the Tachiyazawa-gawa R. The same is true for the length ratio. 3. Concerning to the quality of water, the main dissolved constituents in the Nikko-gawa R. and the Aisawa-gawa R. are sodium bicarbonate and sodium chloride due to the influx of the oil field brine in the drainage area. Different from these rivers, the dominant dissolved mineral in the Tachiyazawa-gawa R. is calcium sulfate which is supplied with the upper reaches subjected to the solfataric action of the Gassan Volcano. 4. According to the yearlong observation of the quality of water, the mean annual concentrations of Na^+, K^+, Cl^- and SiO_2, are in the following order: the Aisawa-gawa R. ≧ the Nikko-gawa R. ≧ the Tachiyazawa-gawa R. But in the case of Ca~<2+> and S0^<2->_4, the different order is found to exist: the Tachiyazawa gawa R > the Aisawa-gawa R. > the Nikko-gawa R. 5. The quality of water of the rivers has been statistically kept constant for these thirty years. 6. The transportation of total dissolved minerals and individual constituents in the streams was observed in 1979 fiscal year. A simple relationship is found to exist between each mineral discharge (Q_d) and flow rate (Q): Q_d=aQ^n, where a and n are constants dependent on the ion and the river. Regarding the same ion, the n value of the Nikko-gawa R. is nearly equal to that of the Tachiyazawa-gawa R. 7. Annual total discharge per unit drainage area (km^2) is estimated at 100 tons for the Nikko-gawa R. and the Tachiyazawa-gawa R. and 200tons for the Aisawa-gawa R. The annual amount of transportation of individual constituents is in the order: Na^+<Ca^<2+>, HCO^-_3>C1^->SO^<2->_4 for the Nikko-gawa R. and the Aisawa-gawa R. ; Ca<2+>>Na^+, HCO^-_3>S0^<2->_4>Cl~- for the Tachiyazawa-gawa R.