Seasonal runoff—sediment dynamics and its controlling factors were studied in an agricultural watershed in the black soil region of Northeast China. Daily sediment and discharge data from 1957 to 1989 (except for 1961 and 1962) was used to investigate runoff-sediment dynamics, and the observed data shows that the discharge patterns are dominated by runoff from July to September, which accounted for about 64.7% of annual discharge; fluctuations in suspended sediment concentration (SSC) are markedly different from discharge fluctuations; and SSC in the snowmelt season (April) and late June to July is conspicuously higher than at other periods of the year. One concept was proposed to isolate the individual effect of each controlling factor on SSC from their joint effects and the preliminary analysis shows that: (1) high SSC in April is mainly dominated by freeze—thaw, and high SSC in July is dominated by intensive rainfall erosivity rather than volume of discharge; (2) an increase in rainfall erosivity increases SSC whether or not a field is covered by crops; (3) the effect of rainfall erosivity with increasing SSC in July is larger than the reduction effect from crop cover. The results reveal that the concept of isolating an individual effect from the joint effect played by multiple controlling factors on SSC provides potentially an efficient and effective way for evaluating land use and management practices in a new area with limited data.