Among various types of bed forms in alluvial channels, many researchers paid much effort to clarity the condition of the formation of bars, since the hydraulic characteristics of natural streams cannot be fully understood without a knowledge of basic principles that govern the formation of bars. To date, however, there has been no satisfactory explanation for the formation of various types of bars. Through the flume experiments, the author recognized 4 types of bed configuration de-fined by bar development, each of which appears according to the different hydraulic regimen (Ikeda, 1973). In this paper, the author attempts to classify the configuration of alluvial channels into these 4 types and examine the condition of bar formations, using the same parameters as in the flume experiments. The lower Omoi River, Tochigi Pref., was chosen for a study area where a detailed field observation was made along 4 reaches (from No. 1 to No. 4 in down-stream direction, Figs. 1 A, 2 B) which represent above mentioned 4 types of bed configu-rations respectively. No. 1 reach: corresponding to Type 1; The type of multiple bars, consisting of a set of linguoid and diagonal bars, is generally regarded to be features of channels of low sinuosity, with high width-to-depth ratio and steep slope. At low flow, higher parts of each bar emerge as mid- and side-shoals, dividing the stream (Fig. 5). These bars correspond to braid bars (Allen, 1968), spool bars (Krigstrom, 1962) and channel bars (Thornbury, 1954). Figures 10, 11 and 18 show the features of similar channel configuration. No. 2 reach: corresponding to Type 2; The type of alternate bars, consisting of diagonal bars with sharp crest lines which migrate downstream (Fig. 6). A sharp avalanche face of their downstream edge and abrupt change of talweg from one side to the other constitute a characteristic feature of this type of bed configuration. At low flow, because of the alter-nate arrangement of side shoals, alternation of riffles and pools are seen. No. 3 reach: corrresponding to Type 3; The type of alternate bars, consisting of diagonal bars with obscure crest lines, often arises in channels of sandy bed. It resembles to type 2 in plan (Fig. 7), but relief of bars is much smaller, therefore riffles and pools do not appeareven at low flow, and talweg meanders continuously from one bank to the other. No. 4 reach: corresponding to Type 4; No bars are formed. Although alternate de-pressions are formed at low flow even in the straight No. 4 reach (Fig. 8), the maximum relief in the bed is less than 50 cm. It is regarded, therefore, that bars are not created in such channels of gentle slope and very small width-to-depth ratio, as channels of delta plain (See also Figs. 12 & 14). The median grain size of bed materials in the upper part of the study area is 10_??_20 mm in diameter, while at 14 km below the junction of Sugata River, it decreases suddenly to 1 mm (Fig. 2 B). And the channel slope decreases downstream in accordance with the change of grain size of bed materials mentioned above (Fig. 2 A). Further, the channel configuration of the lower Omoi River also changes downstream accordantly to the decrease of channel slope and grain size of bed materials (Figs. 3 & 4). This relation is schematically illustrated in Fig. 9. In order to clarify the hydraulic conditions of formation for each 4 types of bed configu-ration, two parameters, U*/ U*c (flow intensity) and S•W/D (channel form index) were obtained by dimensional analysis based on similarity-law of distorted models. Here U* is shearing velocity at bed, U*c is critical shearing velocity for bed materials, S denotes slopes, and as the channel forming discharge is here approximated by the bankfull discharge, the bankfullwidth and -depth are used for W and D respectively (Table 1).