以歷史圖資探究二十世紀初期嘉南平原河道交織的近自然狀態與人為影響

Reconstructing the Near-Natural River Network of the Early 20th Century and Examining Human Modifications in the Chianan Plain Using Historical Maps, Southwestern Taiwan

長期以來，氾濫平原是人類重要的生存空間，但居民同時也須承受洪水所造成的災害風險。因此，若可判釋與解讀存留在地景中河流作用的痕跡（小尺度河流地形），將有助於掌握河流在最極端情況下的影響範圍，而有利於因應未來氣候變遷下極端事件可能造成的洪災。過去臺灣的河道變遷研究，多僅專注於主流河道，本研究以臺灣嘉南平原（北港溪至二仁溪）河流為對象，嘗試還原平原上河道的近自然狀態，並釐清在人為工程影響下，主、支流河道在百年間發生的變遷。本研究利用數值高程模型及歷史圖資進行數化及疊圖分析，建置包括河道及水體、洪水流路、地形崖、堤防等氾濫平原小地形或地物的空間資訊資料庫，同時判釋氾濫平原地形分區，重現嘉南平原在20世紀初較接近自然狀態下的河流地形樣態。
嘉南平原的各條河流在近百年內陸續被渠道化，現今多被視為各自獨立的水系。本研究則發現在20世紀早期人為影響較小時，嘉南平原上河道密布，主、支流間複雜交織，而且有些主流的擺移範圍超過現今河堤位置。支流的變化方式上、下游段不同，位在較上游（東側）之沖積平原上的許多水體發生破碎化、埤塘化；位在較下游（西側）之海岸平原上河道則多發生截彎取直式的渠道化、濕地排水等改變。此外，判釋歷史地圖上連續水體、不連續但具方向性的殘存水體以及地形崖的結果可知，現今相鄰河流之間在過去常有河道連結、互相影響的狀況。本研究透過河流微地形證據所建構出嘉南平原上各河流影響範圍與相互關係，提供了洪水管理的重要基礎環境資訊。

Floodplains have long provided vital living space for human settlements, yet they also expose communities to the risk of flood-related disasters. Understanding and interpreting the traces of past river activity—such as river microtopography—can help delineate the extent of river influence under extreme conditions. This is especially crucial for responding to floods that may be exacerbated by climate change. In Taiwan, most previous research on river channel changes has focused primarily on major rivers. In contrast, this study examines the floodplain river systems on the Chianan Plain of southwestern Taiwan, aiming to reconstruct the near-natural state of the interwoven main and tributary channels and to analyze their century-long transformations under the influence of human engineering. Using historical maps and digital elevation models (DEMs), this study digitizes and overlays spatial data to create a comprehensive database encompassing river channels, flood paths, water bodies, riverbank cliffs, river dikes, and other microtopographic features. These data are used to reveal the channel characteristics on the Chianan Plain in the early 20th century.
The findings show that river channels in this region have been progressively channelized and regarded as the distinct drainage systems. In the early 20th century, before significant human modification, river channels were densely distributed across the plain, with some main channels migrating well beyond the limits of current river dikes. Tributaries exhibit different patterns of change in upstream versus downstream areas: fragmented, pond-like water bodies dominate the alluvial plain, while the downstream coastal plain features more channelized flows and wetland drainage. By interpreting continuous water bodies, discontinuous yet directionally aligned remnants, and topographic cliffs preserved in historical maps, the study reveals frequent interactions among adjacent rivers.
This research reconstructs the spatial extent and interrelationships of river channel systems in southwestern Taiwan through evidence of river microtopography. The results provide a critical foundation for understanding historical fluvial dynamics and for informing contemporary flood risk management and planning.