The YRD (36° 24.0′–36° 48.2′ N; 117° 59.8′–120° 36.1′ E) is situated to the south of Bohai Bay in eastern China ( Figure 1 ). It is not only the youngest delta in China’s warm temperate zone but also the largest delta with the most complete preservation (Song, 2015). The case study area is representative of coastal wetlands in the high-efficiency eco-economic zone of the Yellow River Delta, which extends from the mouth of the Taoer River in Zhanhua to the Qimu Cape in Longkou. Most areas of the YDR are dominated by irregular semidiurnal tides, while the tide in areas from the north of Doying Port to the south of the current course of the Yellow River is a diurnal tide (Yang et al., 2015; Weikang et al., 2018). The Yellow River has changed its channel courses multiple times, and the deposition of new subdeltas and the erosion of abandoned subdeltas have occurred successively (Zhang et al., 2018). In the study area, the mean tidal range on the north and south coasts is about 1–2 m, while that on the east coast near the current course is 0.76 m and that on the south coast is 0.61–1.68 m (Yang et al., 2015; Zhou et al., 2016; Wang et al., 2017). Changes to the coastline on the north coast were influenced by reclamation activities in recent decades, with maximum expansion rates up to 123.98 km² (Fan et al., 2018).

As the drainage area of one tidal creek is much smaller compared to the entire YRD, there is no need to optimize the entire resistance surface of the YRD each time, especially considering the huge operand of the genetic algorithm. According to different hydrological conditions, patterns of human activity, and natural resources, we divided the entire area into six zones ( Figure 1 ). The different hydrological conditions were mainly due to the change of courses of the Yellow River into the sea in different periods in history. Zone 1 was located on the northwest coast of the study area and mainly contains the Ancient YRD due to the flow of the Yellow River into the sea in 1855 through the Taoer River. Zone 2 mainly contains the delta lobe of the Diaokou course, has suffered significant erosion since it was abandoned in 1976. Zone 3 was the subdelta of the Yellow River that flows into the sea in Stream Shenxian and courses south and north of it from 1953 to 1976. Zone 4 contained the National Reserve of the YRD, which was built in 1992 to protect coastal wetlands. Zone 5 contained the southern abandoned subdelta formed at the beginning of this century from the south of the current course of the Qingshui Sream to the southern plain of Laizhou Bay. Zone 6 mainly contained Laizhou Bay.

To understand the changes in the hydrological connectivity of tidal flats in the YRD, we extracted tidal creeks from Landsat images over the past 35 years at 5-year intervals. Level-1 Precision and Terrain (L1TP) data of Landsat-5 Thematic Mapper (TM) and Landsat-8 Operational Land Imager (OLI) were available from the United States Geological Survey (USGS) Earth Explorer (Survey, U.S. G., 2022). Images with a cloud cover of less than 10% were chosen. All images were pre-processed by ENVI 5.3. Radiometric calibration and Fast Line-of-Sight Atmospheric Analysis of Spectral Hypercubes (FLAASH) were used to eliminate the errors from sensors, solar radiation, and the atmosphere. (The information from the images, as well as the specific year chosen, are shown in Table 1 ).

To calculate the hydrological distance between specific sites, we focused on the center line of tidal creeks. Considering that the small creeks (e.g., first order according to the Horton–Strahler method) were difficult to extract from tidal flats using the automatic methods, visual interpretation was employed for creek extraction. Considering the strong impact of tidal conditions on the appearance of tidal creeks in the images, we used the nonstandard false-color image to assist with the extraction. Because this combination of bands could better distinguish the boundaries between water bodies and land, it could help identify tidal creeks from tidal flats. Command of change RGB bands in ENVI 5.3 was used to execute the band combination of near-infrared bands, short-wave near-infrared bands, and red bands (453 and 564 for TM and OLI, respectively). The combination emphasizes water, which is often used to distinguish the boundaries between water and land in the coastline and tidal flat surveys.

The landscape features of sea reclamation activities in the YRD were obtained from the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, to represent the resistance surface in the YRD. The sea reclamation activities in this dataset include categories of city construction land, port, industrial land, dry farmland, forest, bare land and tourism land, grass, salt pans, mariculture ponds, rivers, and reservoirs. On this basis, we reclassified these features into four categories, according to their similarities. City construction land, port, industrial land, dry farmland, forest, bare land, tourism land, and grass were classified as reclaimed land. Salt pans and mariculture ponds were classified as sea enclosure activities. Rivers, reservoirs, and offshore water were classified as freshwater resource facilities. Considering the high intensity and large temporal and spatial scale of oil exploration in the YRD, we also identified engineering in oil fields, including oil wells, roads leading to different oil wells, and oil fields that occupied large areas of land. In addition, coastal salt marshes and tidal flats were classified as coastal wetlands. All such surface data were processed into a raster stack to accommodate the requirements of the algorithm. The constructed resistance surface contains both a single and a combination of the sea reclamation activity categories (composite resistance surface).

The landscape resistance inversion results of tidal creeks with changes in 5-year intervals that were significantly related to landscape resistance revealed that resistance surfaces and the number of tidal creeks were different in different zones and at different times. Only resistance surfaces containing sea reclamation activity categories (with R ² higher than 0.1) are shown as the leading resistance surface in the rank of all possible resistance surface combinations in each tidal creek ( Table 2 ) (the model result with optimal resistance surfaces composed of only coastal wetland of saltwater, null model, and distance model is not listed in the table). In general, the period of 1990–1995 had the greatest number of tidal creeks influenced by resistance surfaces, followed by the period of 2000–2005. The creek ID in each zone demonstrates that the influence of resistance surfaces at different times can be seen in the same tidal creeks. The majority of resistance surfaces are composite resistance surfaces, with two surfaces of either human activity or natural elements. Based on the specific resistance surfaces identified in Table 2 , we discovered that sea enclosure activity played a significant role in the formation of resistance surfaces in the majority of zones, as such activities were present in the majority of resistance surfaces in zones 1, 3, 4, 5, and 6. Freshwater resource facilities are the second surface resistance component since these facilities were involved in the majority of zone 2 and 6 resistance surfaces. Engineering in oil fields also contributed to zone 4 resistance surfaces.

The result of the spatial-temporal distribution of resistance values in the YRD showed that high and low resistance values were distributed in different areas in different periods ( Figures 3 – 5 ). According to the intensity of human activity and the invention time, the six zones were divided into three groups for analysis. Zones 1, 2, and 5 were grouped together because their human activities began at about the same time in the 1990s ( Figure 3 ). Zones 3 and 6 were grouped together as their human activities began the earliest (and earlier than the time scale in this study) ( Figure 4 ). Zone 4 was a single group with supposed minimal human intervention as the National Natural Reserve of the YRD is located within it ( Figure 5 ). The findings revealed two types of connections between landscape elements and resistance values. One is that a single landscape feature had a single resistance value, such as mariculture in zone 1 with a resistance value of 0.12 in 1990–1995 in tidal creek 1 ( Figure 3A ). Another is in the form of component features (CF), such as the CF-1 in zone 1 with a resistance value of 0.07 in tidal creek 2 in 1990–1995 ( Figure 3A ). CF-1 contained reservoir, mariculture, salt marsh, and tidal flat landscape features ( Figure 3A ). In the case of the former, each landscape feature had a single resistance value, therefore we referred to them as independent features. By comparison, for the latter, as multiple landscape features had a single resistance value, we termed them composite features. According to the circuit theory, higher resistance values might arise from attributes of landscape features or the distribution of the landscape features (McRae, 2006). From this point, independent features might be more likely to reflect the influence of landscape feature attributes than the same landscape features with the same resistance value. In addition, composite features might be more likely to reflect the influence of the distribution of the landscape features, as distinct landscape features in them did not make the resistance values different.

Agriculture (0.91), reservoir (0.92), and mariculture (1.00) in zone 5 were the independent feature types with the highest resistance values in zones 1, 2, and 5 ( Figure 3 , the initial stage of human activities) ( Figure 3C ). In zone 1, CF-4 (0.94) has the highest resistance value in composite features ( Figure 3A ). Other high values in resistance surface, both independent and composite features, are around or lower than 0.5. In zones 3 and 6 ( Figure 4 , the mature stage of human activities), the independent feature types with the greatest resistance values arise in the river (0.97) in zone 6 ( Figure 4B ) and mariculture (0.67) in zone 3. The highest resistance values in composite features were found in CF-3 (0.58) in zone 6 ( Figure 4B ) and CF-1 (0.50) in zone 3 ( Figure 4A ). Other high values in resistance surface, both independent and composite features, are around or less than 0.7. In zone 4 ( Figure 5 , reserve), the independent feature types with the highest resistance values appear in mariculture (0.78). Other high values on the resistance surface, primarily independent features, are less than 0.5. The results demonstrate that in regions with early phases of human activity, several specific features play key roles in the form of resistance, and they have an exceptionally high value. In comparison, the average value of landscape features was higher in places with mature stages of human activity than in areas with early stages of human activity.

The spatial distribution and their changes in the length of impacted tidal creeks by sea reclamation activities (influenced tidal creeks, for short) revealed that the tidal creeks in different zones had varied characteristics ( Figure 6 ). Tidal creeks in zones 1 and 2 had the most lost edges, and their mean length was significantly longer than in other zones ( Figure 6A ). Tidal creeks in zone 4 had more edges that had changed length (increased or reduced) ( Figure 6D ). Tidal creeks in the other three zones all revealed a pattern of only one tidal creek changing significantly over a 5-year interval and other tidal creeks changing somewhat in edge number or edge length ( Figures 6B, C, E, F ).

Tidal creeks impacted by resistance surfaces (influenced tidal creeks, for short) were distributed across the Yellow River Delta near abandoned estuaries from the Yellow River’s previous course. The Yellow River has changed its course thousands of times due to accumulative landform (due to significantly more sediment and less water in the river), resulting in juxtaposed and imbricated deltaic lobes around the river mouths at different times (Zhang et al., 2018). The Taoer River estuary was the source of the most impacted tidal creeks in zone 1 from 1990 to 1995. Tidal creeks have formed in the Taoer River estuary since the Yellow River left. Because the Taoer River was one of the first routes that the Yellow River took to reach the sea (around 1904–1917), tidal creeks in the Taoer River estuary have developed for a long time. In zone 2, the Tiao River estuary had the greatest two impacted tidal creeks in the 1990–1995 and 1995–2000 periods (the Tiao River was the course that the Yellow River used to flow into the sea in 1917–1925). The three impacted tidal creeks in zone 3 were near the Stream Shenxian, which is the course that the Yellow River used to take into the sea in 1953–1960. The Stream Tianshui estuary in zone 4 had the most impacted tidal creek (the Stream Tianshui was the course that the Yellow River used to flow into the sea around 1934–1953). Another impacted tidal creek from the years 2000 to 2010 was in the Stream Qingshui estuary (the Stream Qingshui was the route that the Yellow River took to reach the sea from 1976 to 1996). Tidal creeks of the Zhimai River estuary were impacted in zone 5 (the Zhimai River was the course that the Yellow River used to flow into the sea in 1929).

Resistance to movement values provided a quantitative estimate of how landscape features or environmental parameters affect movement. “resistance” refers to an organism’s willingness, costs, or friction when traveling or moving on a surface (McRae, 2006; Zeller et al., 2012). Resistance is utilized in this study to represent the degree to which sea reclamation activities impede or promote water flow movement. The resistance surfaces of various tidal creeks with more than one impacted period varied at different periods. This finding was related to the rate of development of the YRD in the most recent 30 years. Specifically, mariculture activities in zones 1, 2, and 5 (in Zhanhua, Hekou District, and Dongying District) began in the mid-1980s, and generally obtained better yield and expanded the active area in the mid of 1990s after experiencing some technical difficulties and mariculture management measure adjustments (Committee, 2002; Committee, 2008; Committee, 2010). Consequently, mariculture has not yet become a resistance surface in these zones over the periods of 1985–1990 and 1990–1995. During that time, freshwater resource facilities served as resistance surfaces. Storm surges began to harm people and property more regularly as mariculture began to thrive in the tidal flats. When large-scale mariculture activities began after 1995, dikes and hydraulic engineering along the mainstream and near the estuaries of the rivers were built to higher engineering standards (Committee, 2010). As a result, alterations in tidal creeks may occur at the start of resource-use activities. In other words, resource utilization activities may serve as driving forces for the creation of resistance surfaces.

The spatial distribution of influenced tidal creeks exhibited a pattern related to tidal creek evolution. In the first five zones, the impacted tidal creeks, which were generally large-scale tidal creeks, were distributed near estuaries along previous courses of the Yellow River ( Figure 6 ). Taking zone 1 as an example, as far from the Yellow River’s current estuary (the Stream Qingshui estuary in zone 4), it has been affected only minimally by sediment from the recent course of the Yellow River, and the coastline in this area has changed slowly. As a relatively stable condition for tidal creek development, tidal creeks were thus extremely developed before 1985, when dike constructions began (Huang and Fan, 2004). Similarly, for zones 2 and 5, tidal creeks began developing when human activity began (Committee, 2002; Committee, 2008). The existence of increased and newly developed edges also revealed that many influenced tidal creeks were still developing. Thus, the formation of resistance was not only due to human activity but also the natural evolution of tidal creeks was an important aspect in the early stages of resistance formation.

The results of different resistance values of human activities influencing hydrological connectivity revealed that the same type of human activity could have different influences at different times and spaces. The resistance value depended on the intensity of the human activity itself, for example, the area of sea reclamation activities. During approved sea reclamation activities in the YRD, strict classification has been made to regulate different human activities (Zhou, 2017; Liu, 2020). For example, in Shandong province, for reclamation projects that change the property of the sea completely, such as port and dike, projects with areas between 60 and 100 hm² should be applied to provincial government agencies (Administration, 2008). By comparison, projects that did not change the property of the sea with areas between 500 and 700 hm², such as bathing beaches, should be applied to the provincial government (Administration, 2008). However, the classification for each project was only based on the properties of each human activity. Although the impacts of sea reclamation activities on ecosystems have been assigned to environmental impact assessment the environmental impact assessments for reclamation activity projects have considered the impacts on the environment in aspects of hydrodynamic, water quality, landforms, oceanic sediment, and so on, it does not take hydrological connectivity of tidal creeks into consideration in the indexes. More comprehensive regulatory approval steps may be needed to take the impacts of the project on spatially explicit tidal creek connectivity into the evaluation system to classify projects, rather than the current criteria that only focused on project attributes. One of the suggested ways could be that resistance values related to tidal creek connectivity could first be calculated for each land use zone in the planning stage, which could make into a single resistance map. In the steps to check the location of the project in the planning map, the resistance zone of the project could be identified according to its location. Thus, the attributes to classify projects could include project types, area, as well as its resistance value.

The relationship between tidal flat evolution and resistance surface development revealed that there were several previous estuaries with the potential to develop large-scale tidal flats and become stopover hotspots along shorebirds’ migratory routes. However, the government of these estuaries has not utilized the ecological value of tidal flats to both benefit local government and regional ecological safeguards. Instead, all the governments of the estuaries focused on resource development activities, for example, mariculture activities, to increase fiscal revenue. By comparison, in zone 4, as large-scale tidal flat and coastal salt marsh ecosystems are well preserved, developing tourism is chosen by the local government, which has also proved to be a good way to both increase fiscal revenue and maintain the ecosystem’s benefits. Thus, a good understanding of the potential of resources within the jurisdictional zones is necessary for local governments in the YRD to choose the best way for sustainable development. This required systematic monitoring of the ecosystems and sufficient conservation awareness. Although human activity has caused the most tidal flat loss, except for that in the National Reserve of the Yellow River Delta (Shi et al., 2016; Chen et al., 2019), local areas with developing tidal creeks still exist, including, for example, the Zhimai River estuary. Despite the fewer areas for conservation in regions such as the YRD, which has severe human activity, attention could also be paid to restoration, for example, returning mariculture to tidal flats (Zhang, 2022). According to the potential tidal and hydrodynamic conditions, further tidal flat development may also exist in some areas.