Water resources, the social economy, and the ecological environment are interrelated and interacting complex systems, and the relationship among them affects the sustainable development of a region. To explore the interactive relationship and driving factors between water resources, the social economy, and the ecological environment, the Yellow River Golden Triangle region is taken as the research object in this paper. By constructing a coupling-coordination evaluation index system of water resources, the social economy, and the ecological environment system, the coupling-coordination development of this region from 2011 to 2021 is studied using the coupling-coordination degree model, and the influencing factors of coupling-coordination development are identified by gray relational analysis. The results show that from 2011 to 2021, the comprehensive evaluation index of the water resources, social economy, and ecological environment in the Yellow River Golden Triangle region shows a trend of steady development followed by a gradual increase. The water-resources subsystem restricts the development of the coupling system. The coupling-coordination degree increased from a barely coordinated stage in 2011 to a well-coordinated stage in 2021. The social economy subsystem and water-resources subsystem are the main factors affecting the coordinated development of the coupling system.

The floodplain transverse slope is a significant parameter reflecting the degree of development of a secondary suspended river, as well as a crucial index of the flood risk in the river channel. Clarifying the factors that influence the evolution of the floodplain transverse slope has always been a hot and difficult topic for researchers working on the Yellow River management. We took the severe section of the secondary suspended river from Dongbatou to Gaocun in the lower Yellow River as the research object, selecting the annual runoff, annual sediment load, annual sediment coefficient, and the intensity of flood-season flow scouring at the Huayuankou station in the downstream as the water–sediment indexes. The correlation between different water–sediment indexes and the floodplain transverse slope under three modes: interannual, flood season, and flood-season overbank was studied through methods such as cross-wavelet transform and wavelet coherence analysis. The results showed that under the three modes, the annual sediment load and annual sediment coefficient had a high correlation with the evolution cycle of the transverse slope, followed by the intensity of flood-season flow scouring, and the annual runoff had the lowest correlation. Meanwhile, the change in the transverse slope had a good correlation with the flood-season overbank mode, indicating there was a high similarity between the water–sediment characteristics of floodplain flooding and the evolution cycle of the transverse slope; that is, the change in the transverse slope is greatly influenced by floodplain flooding events.