## 1 Introduction  
A nuclear reactor accident occurred at Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in Japan on 11 March 2011, causing massive radioactive contamination [1; 2; 3; 4; 5; 6; 7] in Fukushima and neighboring prefectures. The short-lived radionuclides rapidly decayed out, but radionuclides \({}^{137}\)Cs (half-life = 30.1 y), \({}^{134}\)Cs (2.06 y) and \({}^{90}\)Sr (28.9 y) retained rather longer times in the environment. The total release of \({}^{137}\)Cs and \({}^{134}\)Cs from the FDNPP were estimated to be 24 PBq, which was lower than 132 PBq in the case of the Chernobyl accident [1]. The ratio of \({}^{90}\)Sr/\({}^{137}\)Cs was estimated lower than about 1/1000 [1; 2]; thus \({}^{137}\)Cs and \({}^{134}\)Cs are the main radionuclides to be traced in the environment. Radiocesium (\({}^{137}\)Cs + \({}^{134}\)Cs) deposited on the ground quickly binds to fine soil particles, as reported by [PERSON] and [PERSON] [8] and [PERSON] et al. [9]. The transfer process of radiocesium from forest to river environments was reviewed by [PERSON] et al. [10] and they suggested that soil erosion by rainfall is one of main process of the radiocesium transfer. River surveys in Fukushima were performed by many authors. [PERSON] et al. [11] measured the transport of radiocesium through two rivers (Hiso and Wariki Rivers) in the Iitate Village. The discharge of radiocesium was estimated to be 0.3% and 0.5% of total amount of radiocesium deposition on the catchments of these rivers, respectively. [PERSON] et al. [12] studied the concentrations of particulate/dissolved radiocesium in river water at 30 sites in Fukushima and Miyagi Prefectures in December 2012, and evaluated the solid/liquid distribution coefficients of \({}^{137}\)Cs. [PERSON] et al. [13] and [PERSON] et al. [14] studied the particle size dependence of radiocesium concentration in river water and riverbed sediments along the Abukuma River. [PERSON] et al. [15] conducted numerical simulations of radiocesium distributions in the Abukuma river systems. [PERSON] et al. [16] studied deposition of radiocesium on the floodplains along two rivers (Ukedo and Odaka Rivers) in Fukushima. [PERSON] et al. [17] measured temporal changes of dissolved \({}^{137}\)Cs concentrations in groundwater and stream water in Yamakiya District in Fukushima. [PERSON] et al. [18] made a simulation concerning to the behaviors of suspended sediments and \({}^{137}\)Cs at the Ogaki Dam reservoir on the Ukedo River, Fukushima, during the heavy rainfall. [PERSON] et al. [19] collected suspended sediments, filtered water, and bottom sediments from six coastal rivers in Fukushima during low-flow conditions in November 2013. They derived significant correlations between radiocesium concentration and radiocesium inventories, liquid-solid partition coefficients K\({}_{\text{D}}\), and significant radiocesium transfer from contaminated forest litter to downstream rivers. Up to now, information about the riverine radiocesium contamination in Fukushima was extensively accumulated as described above. However, long-term monitoring concerning the transport mechanism and temporal variation of radiocesium from land to river and from upstream to river mouth are needed for further research. After the Chernobyl accident, long-term radiological monitoring on riverine radiocesium contamination were conducted for five years or more in Ukraine [20; 21; 22], five rivers in Finland [23], and one river in Italy [24] and other European countries. Minami-Soma City is located approximately 10-40 km to the north of the FDNPP, as shown in Figure 1. The eastern area on the coast of the Pacific Ocean was severely damaged by the tsunami, and the western area of the city joins the Abukuma mountain range, which was subjected to high levels of radioactive contamination. Most of the rivers in the Minami-Soma City originate from the Abukuma mountain range. Expansion of radiocesium contamination through the rivers is one of the serious concern of residents.