Leaving a plastic legacy: current and future scenarios for 4 mismanaged plastic waste in rivers* 5

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Plastic pollution in our environment has received considerable attention in recent years given 42 its potentially hazardous impact on ecosystems, human livelihoods and economies. However, 43 the majority of existing plastic research has focused on the marine environment (Galgani et   One of the main motivations of the current study is to highlight the legacy of MPW in different 89 river systems. To achieve this goal, we classified each river system according to morphology, 90 the historical river migration of the system (1984-2020), and the anthropogenic impact. an inactive river channel as any location where 90% of pixels in the Landsat image archive at 105 a given locality were classified as water over the 36 years of available imagery. Any location 106 where a given pixel changes from water to land (or vice versa) with at least 2 years of 107 observation defines an area of an actively migrating river system. This map receives a reported 108 commission accuracy over 98.3% for waterbody extent (Pekel et al., 2016). Lastly, we consider 109 the impact of humans on river systems by using the free-flowing river (FFR) dataset (Grill et    To relate the input of MPW to each river system, we first calculate the total volume of MPW  We classify river systems based on eight categories that define: 1) the morphology of either 198 meandering or braided rivers, 2) an actively migrating river system, and 3) whether the river is found in Asia, followed by 14% in Africa and 6% in South America. Europe, North America, 228 and Oceania together amount to less than 4% of the total MPW input. Based on a 'business-as-229 usual' projection, the volume of plastic input to rivers will significantly increase from 14% of MPW input, respectively. Based on a business-as-usual projection, most river system 252 types will see an increased MPW input by at least 2.5times. However, the data suggest that due 253 to the current distribution of river systems, migrating braided river systems will be the most The potential accumulation of MPW as of 2015 has impacted 84% of rivers by surface area. In 288 summary, we estimate that 47% of the surface area of rivers is exposed to a low risk of MPW,

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17% to medium, 13% to high and another 7% to very high risk ( Figure 6A). By 2060, a 290 business-as-usual projection will mean very high MPW impacted rivers will increase to 11% 291 ( Figure 6). In addition, medium and high MPW impacted rivers will continue to remain 292 significant covering roughly 14% each of the total river surface area. In total, no less than one 293 third of all rivers will be exposed to a medium or higher amount of MPW by 2060 given the 294 current trends in plastic waste.  If plastic waste management is improved by a global effort (scenario 2060B), the number of 303 very high MPW impacted rivers will decrease significantly to 4.8% and high MPW impacted 304 rivers will reduce to 7% ( Figure 6A). Lastly, improved waste management and recycling 305 strategy (scenario 2060C), will result in more than a 50% decrease in medium or higher levels 306 of MPW exposure in river systems. Very high MPW exposure will decrease 2.2 times and high 307 MPW exposure will decrease nearly 3-fold compared to the present-day, Figure 6A. In contrast to the input of MPW (Figure 3), the accumulation of MPW will impact a larger area 310 of river systems globally. In 2015, 35% of the total area of rivers exposed to a medium or higher 311 amount of MPW are found in North America, Europe and in South America ( Figure 6B). Asia, 312 however, remains the most affected with 47% of all impacted river systems whereas Africa 313 represents 16% by river surface area. By 2060, Scenarios B and C will have the most positive 314 change in North America, Europe and South America, whereas Africa will increasingly become 315 the most impacted region by river surface area at 38%.

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Proportionally, the type of rivers most exposed to the 2015 distribution of MPW are free-318 flowing migrating meandering rivers representing 34% of the total surface area of rivers ( Figure   319 7A). Within those meandering rivers, 4.9% are exposed to a medium volume of MPW plastic 320 risk, another 4.3% are high and nearly 1.8% are very high. In total, 25% of the total river surface 321 area of free-flowing meandering or braided rivers is exposed to at least a medium amount of 322 MPW risk. While impacted rivers represent a smaller 22% of the total river surface area ( Figure   323 4), 43% of those river systems are exposed to at least a medium amount of MPW.

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Based on future scenarios of medium or higher MPW exposure by river type ( Figure 7B), a 326 2060 scenario C implementation will have the largest impact on rivers currently classified as 327 free-flowing. In particular, we see that the surface area of rivers exposed to significant amounts 328 of MPW in migrating meandering rivers and non-migrating braided rivers is reduced by 5.6 and 329 3%, respectively. However, proportionally, rivers with impacted flow due to human 330 intervention, will have the largest change in MPW exposure. In total, impacted rivers will see 331 a reduction in medium or higher MPW exposure by nearly 3-fold compared to 2-fold of free-332 flowing rivers based on a 2060 Scenario C trend ( Figure 7B). The morphology of a river is an important marker to predict the potential storage of plastics two primary factors: sediment load and river discharge (Figure 8) (Schumm, 1985). These 366 geomorphic river types are potentially very informative with respect to the retention or 367 expulsion of plastic along the river course.

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For example, consider a meandering river system having a low sediment load (Figure 8). The 370 channel does not significantly migrate and hence there is very little sediment accumulation 371 along the river course. Therefore, the river water column (rather than fluvial sediment deposits) 372 will contain most of the MPW that has entered the system. This pollution is not retained in the 373 river but rather it is delivered to the oceans (unless otherwise entrapped by vegetation, aquatic 374 life, dams or other human actions that would retain plastic in the river system).

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On the other hand, a river system with a high sediment load has a much higher likelihood to  The morphology of a river system is also an indication of river behavior with greater water 396 discharge in braided systems compared to meandering rivers (Schumm, 1985). The greater  For an active meandering river system, the water discharge may be similar but mechanical 429 weathering of plastics may be slightly higher due to sediment abrasion. Furthermore, plastic 430 waste stored within the pointbars and floodplains of active meandering rivers will likely have 431 18 increased residence time providing further opportunity for biological and chemical breakdown.

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Actively migrating meandering river systems currently receive 29% of MPW input (or 0.23 433 MT/yr; Figure 3B) and represent 33% of river systems by area that may receive accumulation 434 of MPW ( Figure 7A).

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The higher discharge and energy of braided rivers will expose plastic material to more physical 437 abrasion producing secondary plastic fragments. In inactive braided (or anabranching) river  2060 ( Figure 6A). Here we see that free-flowing actively meandering river systems are the most 465 important, representing 27% of total surface area of rivers globally (Figure 2), and also are 466 likely to see the largest increase in very high MPW exposure (66%, Figure 7B). This suggests 467 that the relative increase in river surface area that will entrap plastic material within the 468 terrestrial environment will see concomitant growth ( Figure 7); thus, the potential direct spatial 469 impact of MPW will be larger, requiring more significant mitigation and remediation measures.  This study has aimed to highlight the different types of river systems currently exposed to MPW 527 and future predictions based on different plastic usage / mitigation scenarios. In conclusion, we 528 find that:

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• Improved recycling and reduced plastic reliance can reduce significant MPW exposure 550 in river systems by as much as 72% in the year 2060. Proportionally the largest 551 difference will be noticed along river systems with impacted flow due to human river 552 management. However, free-flowing rivers will continue to represent the largest surface 553 area of rivers exposed to MPW which will be problematic in any environmental 554 mitigation of plastic pollution given the large area of dispersion.