<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE article  PUBLIC "-//NLM//DTD Journal Publishing DTD v3.0 20080202//EN" "http://dtd.nlm.nih.gov/publishing/3.0/journalpublishing3.dtd"><article xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" dtd-version="3.0" xml:lang="en" article-type="research article"><front><journal-meta><journal-id journal-id-type="publisher-id">OJSS</journal-id><journal-title-group><journal-title>Open Journal of Soil Science</journal-title></journal-title-group><issn pub-type="epub">2162-5360</issn><publisher><publisher-name>Scientific Research Publishing</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="doi">10.4236/ojss.2023.1312024</article-id><article-id pub-id-type="publisher-id">OJSS-129687</article-id><article-categories><subj-group subj-group-type="heading"><subject>Articles</subject></subj-group><subj-group subj-group-type="Discipline-v2"><subject>Earth&amp;Environmental Sciences</subject></subj-group></article-categories><title-group><article-title>
 
 
  The Role of Rivers in the United States 19th Century Territorial Expansion from the Mississippi River to the Pacific Ocean
 
</article-title></title-group><contrib-group><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Kenneth</surname><given-names>Ray Olson</given-names></name><xref ref-type="aff" rid="aff1"><sub>1</sub></xref><xref ref-type="corresp" rid="cor1"><sup>*</sup></xref></contrib></contrib-group><aff id="aff1"><label>1</label><addr-line>Department of Natural Resources and Environmental Sciences, College of Agricultural, Consumer, and Environmental Sciences, University of Illinois, Urbana, USA</addr-line></aff><pub-date pub-type="epub"><day>08</day><month>12</month><year>2023</year></pub-date><volume>13</volume><issue>12</issue><fpage>517</fpage><lpage>533</lpage><history><date date-type="received"><day>21,</day>	<month>November</month>	<year>2023</year></date><date date-type="rev-recd"><day>5,</day>	<month>December</month>	<year>2023</year>	</date><date date-type="accepted"><day>8,</day>	<month>December</month>	<year>2023</year></date></history><permissions><copyright-statement>&#169; Copyright  2014 by authors and Scientific Research Publishing Inc. </copyright-statement><copyright-year>2014</copyright-year><license><license-p>This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/</license-p></license></permissions><abstract><p>
 
 
  This study explores the key role of rivers in the westward expansion of the United States during the 19th century. In the early 1800s, President Jefferson envisioned a United States that extended from the Atlantic to the Pacific Ocean. At the time, the entire United States territory was located between the Atlantic Ocean and the Mississippi River. Much of the land west of the Mississippi River was claimed by Spain, France, or Canada. In 1803, President Jefferson was able to purchase the Missouri River watershed from France via the Louisiana Purchase. This allowed the United States to extend its land claim west from the confluence of the Missouri and Mississippi Rivers to the head waters of the Missouri River at the continental divide in the Rocky Mountains. President Jefferson commissioned William Clark and Meriwether Lewis, in 1803, to explore, discover and describe the Missouri River watershed and find a water route to the Pacific Ocean. The Lewis and Clark Corps of Discovery trip found no such waterway link but did continue to explore the Pacific Northwest lands north and west of the continental divide. The only way that the Pacific Northwest could be claimed as part of the United States was for Americans to settle there before the Canadians did. Starting in the 1820s, many Americans traveled via the Oregon Trail to the Willamette Valley (Land of Flowing Milk and Honey) in Oregon. The primary objectives of this study are to document how the United States: 1) extended its land claims west from the Mississippi River to the North American continental drainage divide; 2) established an American claim to the Pacific North West territory; and 3) fulfilled President Jefferson’s vision of a United States extending from the Atlantic to the Pacific Ocean.
 
</p></abstract><kwd-group><kwd>Louisiana Purchase</kwd><kwd> President Thomas Jefferson</kwd><kwd> Oregon Trail</kwd><kwd> Corps of Discovery</kwd><kwd> Sacagawea</kwd><kwd> Missouri River</kwd></kwd-group></article-meta></front><body><sec id="s1"><title>1. Introduction</title><p>Meriwether Lewis and William Clark were tasked by President Jefferson to discover, explore and describe Louisiana Purchase lands, which included the Missouri River watershed [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] and to find a water route to the Pacific Ocean. During their trip, in 1803, Lewis and Clark received official word from President Jefferson that the United States had acquired New Orleans, Louisiana, and 2,144,510 km<sup>2</sup> of the Missouri River watershed from the French [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] . The Louisiana Purchase from France extended the western United States boundary from the Mississippi River to the Continental Drainage Divide at the headwaters of the Missouri River in the Rocky Mountains.</p><p>The Lewis and Clark Corps of Discovery Expedition trip success was a way to encourage Americans to travel west and to settle in the Pacific Northwest. David Thompson, a Canadian, explored most of the Columbia River between 1807 and 1811 for the North West Company. Canadian outposts were later established on the Columbia River at Fort Walla Walla (1818) and Fort Vancouver (1825). The Canadians could not settle the Pacific Northwest, via the Columbia River, before the Americans did, using the Oregon Trail, in the early 1820s.</p><p>The primary objectives of this study are to document how the United States, a country, existing in 1803 from only the Atlantic Ocean to the Mississippi River: 1) extended its land claims west from the Mississippi River to the North American continental drainage divide; 2) established its claim to the Pacific Northwest Territory; and 3) fulfilled President Jefferson’s vision of a United States extending from the Atlantic to the Pacific Ocean.</p></sec><sec id="s2"><title>2. Methodology</title><p>The study shows how the United States methodically extended it territorial claims via rivers from the Mississippi River to the Pacific Ocean. The approach used included the purchase of the Missouri watershed from France via the Louisiana Purchase. In 1803, President Jefferson commissioned William Clark and Meriwether Lewis to explore, discover and describe the Missouri River watershed and find a water route to the Pacific Ocean. The Lewis and Clark Corps of Discovery trip found no such waterway link but did continue to explore the Pacific Northwest lands north and west of the continental divide. Starting in the 1820s, many Americans traveled via the Oregon Trail which paralleled the Platte, Snake, and Columbia rivers to the Willamette Valley (Land of Flowing Milk and Honey) in Oregon.</p><sec id="s2_1"><title>2.1. Cultural and Natural History</title><p>The Spanish and French were the first Europeans to discover the Missouri River and its rich and varied resources in late seventeenth century [<xref ref-type="bibr" rid="scirp.129687-ref2">2</xref>] . The land in the Missouri River basin was originally controlled by Native Americans, Spanish and French. After President Thomas Jefferson agreed to the Louisiana Purchase it became part of the United States.</p></sec><sec id="s2_2"><title>2.2. Exploration of the Upper Mississippi River Watershed and Settlement History</title><p>The French explorers, Pere Jacques Marquette (a Jesuit Priest) and Louis Jolliet, left Lake Huron and traveled down the Wisconsin and Upper Mississippi rivers in May of 1673. These French explorers were the first Europeans to visit the confluence of the Missouri and Mississippi rivers [<xref ref-type="bibr" rid="scirp.129687-ref2">2</xref>] .</p><p>Spain and the young United States signed Pinckney’s Treaty in 1795 which gave America the rights to navigate the entire Mississippi River on its western border [<xref ref-type="bibr" rid="scirp.129687-ref2">2</xref>] . In 1798, Spain revoked the treaty and by 1800, the rights to navigate the Lower Mississippi River (Louisiana) were returned to Napoleonic France. Fearing that the U.S. could be cut off from using the Port of New Orleans and the Mississippi River, President Thomas Jefferson proposed buying the Port of New Orleans [<xref ref-type="bibr" rid="scirp.129687-ref3">3</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref4">4</xref>] for $10 million dollars. France counter offered and indicated it was willing to sell both the Louisiana region and Missouri River valley watershed, for $15 million dollars—a price less than 8 cents per ha. The deal known as the Louisiana Purchase was officially signed in 1803 (<xref ref-type="fig" rid="fig1">Figure 1</xref>) [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] .</p><p>In 1803, President Jefferson commissioned William Clark and Meriwether Lewis to find a water route to the Pacific Ocean and to explore the Missouri River watershed [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] . It was known that the Columbia River system drained west out of the Rockies into the Pacific Ocean and that the headwaters of the Missouri River had a similar latitude as the Columbia River system. Maps of the region were nonexistent or scarce. It was widely thought that there was a connection, or short portage, between the two rivers existed [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref5">5</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref6">6</xref>] (<xref ref-type="fig" rid="fig2">Figure 2</xref>). Meriwether Lewis left Washington D.C. on July 5, 1803, for Pittsburgh, Pennsylvania, via Philadelphia, to obtain a boat for the trip down the Ohio River. He carried a letter from President Jefferson authorizing him to obtain needed support from any federal agency for his expedition. His mission was to find an all-water passage across continental divide and to explore land purchased from Napoleon by President Jefferson.</p><p>Ambrose [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] reported that there were delays in the boat construction. It took until August 31, 1803, for the boat construction to be completed and loaded with supplies for the trip down the Ohio River. Lewis reached the Falls of Ohio at Louisville on October 14, 1803. William Clark joined the Expedition at Clarksville, Indiana Territory [<xref ref-type="bibr" rid="scirp.129687-ref5">5</xref>] . The Corps of Discovery expedition left Clarksville, Indiana Territory on October 26, 1803 [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] . After reaching the confluence of the Mississippi and Ohio rivers at Cairo, the Corps of Discovery expedition then travel north, against the flow, on the Mississippi River to St. Louis. Lewis and Clark set up Camp DuBois in Wood River, Illinois Territory (<xref ref-type="fig" rid="fig3">Figure 3</xref>). In the spring of 1804, Meriwether Lewis and William Clark expedition traveled north, against the current, on the Missouri River.</p></sec><sec id="s2_3"><title>2.3. The Missouri River</title><p>The Missouri River (Big Muddy) (<xref ref-type="fig" rid="fig3">Figure 3</xref>) is the longest tributary of the Mississippi River. The confluence of these two great rivers is located north of St. Louis at Spanish Lake, [<xref ref-type="bibr" rid="scirp.129687-ref7">7</xref>] . The Missouri River is the longest river in North America, even longer than both the Mississippi River and the Yukon River. The Missouri River flows from the western Montana Rocky Mountains southeast to the confluence with the Mississippi River. The Missouri River drains a 14.4 million ha semi-arid watershed. The Missouri River (<xref ref-type="fig" rid="fig4">Figure 4</xref>) provided social, spiritual, physical and economic resources and was a big part of Native American culture. The river became a European route for trade and adventure.</p><p>Lewis and Clark (<xref ref-type="fig" rid="fig5">Figure 5</xref>) started their trip up the Missouri River (<xref ref-type="fig" rid="fig3">Figure 3</xref>) in the spring of 1804. The trip was an attempt find a water route to the Pacific Ocean and to explore the Missouri River watershed [<xref ref-type="bibr" rid="scirp.129687-ref6">6</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref7">7</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref8">8</xref>] . In 1804, Clark and Lewis reached the confluence with the Platte River (<xref ref-type="fig" rid="fig6">Figure 6</xref>). The 1690 km Platte River was not navigable, so Lewis and Clark continued travel north on the Missouri River. President Jefferson previously insisted they stay on the Missouri River. By October (1804) the Expedition passed the Grand River confluence (<xref ref-type="fig" rid="fig3">Figure 3</xref>) where the Arikaras Indians resided. The Arikaras were farmers, who grew grain crops on the alluvial soils, and often traded grain for goods with the Sioux [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] .</p><p>The Corps built a winter camp near a Mandan village (<xref ref-type="fig" rid="fig3">Figure 3</xref>) at a bend in the Missouri River (in present-day North Dakota) (<xref ref-type="fig" rid="fig2">Figure 2</xref>). At Mandan, they met a French Canadian, Toussaint Charbonneau. His 16-year-old wife, Sacagawea (<xref ref-type="fig" rid="fig7">Figure 7</xref>), was from the Shoshone Tribe that occupied land at Three Forks and near the headwaters of the Missouri River. Sacagawea and Charbonneau served as interpreters and translators for the Corps and guided them to the Missouri River headwaters [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] .</p><p>The Corps continued their trip west on the Missouri River in the spring and summer of 1805. Lewis continued to record notes on the diversity of plants and animals observed, such as elk (<xref ref-type="fig" rid="fig8">Figure 8</xref>), pronghorn, and buffalo. The Falls of the Missouri (near Great Falls, Montana) were major obstacles to the Corps’ continued travel on the Missouri River (<xref ref-type="fig" rid="fig2">Figure 2</xref>) and (<xref ref-type="fig" rid="fig4">Figure 4</xref>). A 16 km section of the river had five separate cascades with a combined height of 57 m. They descended an additional 130 m for a total elevation change of 187 m. The 40 km Lewis and Clark portage around the Great Falls of Missouri (<xref ref-type="fig" rid="fig9">Figure 9</xref>) took one month. Lewis and Clark expedition traveled southwest, after the portage, to Three Forks at the headwaters of the Missouri River.</p><p>On July 25, 1805, the Corps of Discovery expedition reached Three Forks of the Missouri River. They had traveled 4000 km on the Missouri River from their Wood River campsite across the Mississippi River from St. Louis. This presented a new dilemma: they needed to pick the correct fork to avoid spending the winter of 1805-1806 in the Rocky Mountains and they did. Sacagawea (<xref ref-type="fig" rid="fig7">Figure 7</xref>), who had been captured five years before, was reunited with her family at the Shoshone campgrounds near Three Forks. Shoshone warriors showed Lewis and Clark the location of the Lemhi Pass and Nez Perce trail that crossed the continental divide.</p><p>Unfortunately, there was no all-water passage across the Rocky Mountains and the continental drainage divide. Having completed both missions assigned by President Jefferson, Lewis and Clark decided to continue to the Pacific Ocean rather than returning home. The Corps of Discovery expedition had to travel by horseback from the continental divide in the Rocky Mountains to the Clearwater River [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] . The Clearwater River was a tributary of the Snake River, which was a tributary of the Columbia River. They used these rivers as gateways to get to the Pacific Ocean [<xref ref-type="bibr" rid="scirp.129687-ref5">5</xref>] .</p></sec><sec id="s2_4"><title>2.4. Snake River Gateway</title><p>The Snake River was controlled by two large Native American groups. The Shoshone occupied the Snake River Plain above and below the Shoshone Falls (<xref ref-type="fig" rid="fig1">Figure 1</xref>0) [<xref ref-type="bibr" rid="scirp.129687-ref9">9</xref>] . The Nez Perce territory stretched from the Columbia Plateau into western Idaho and northern Oregon. The Snake River Plain [<xref ref-type="bibr" rid="scirp.129687-ref10">10</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref11">11</xref>] was a pathway across the main Rocky Mountains for many hundreds of kilometers (<xref ref-type="fig" rid="fig1">Figure 1</xref>0) and (<xref ref-type="fig" rid="fig1">Figure 1</xref>1). As a result of the pathway, the Shoshone had a trade dependent economy with other Native Americans on both sides of the Rocky Mountains. The Shoshone were the most powerful tribe in the Rocky Mountain area and their territory extended across the continental divide and into the Missouri River watershed.</p><p>Most of the Shoshone east of the Rocky Mountains died from a smallpox epidemic brought on by European explorers and fur trappers. The Shoshone eventually merged with other tribes including the Bannock who introduced buffalo hunting on horses, a skill acquired from Europeans. Steelhead and salmon from the Pacific Ocean spawned, by the millions, in the Columbia and Snake rivers and were a vital food supply for the Nez Perce living downstream from Shoshone Falls.</p></sec><sec id="s2_5"><title>2.5. Exploration and Settling of the Pacific North West</title><p>The Lewis and Clark Expedition (1805-1806) was the first American group to cross the continental divide in the Rocky Mountains (<xref ref-type="fig" rid="fig2">Figure 2</xref>). They then traveled down the Snake and Columbia rivers to the Pacific Ocean [<xref ref-type="bibr" rid="scirp.129687-ref7">7</xref>] . The party had crossed the Lemhi Pass, a few kilometers from present-day Salmon, Idaho. Initially they traveled north on the Lemhi River and attempted to descend into the Snake River but found it impassable because of violent rapids [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref2">2</xref>] . They eventually changed pathways and traveled north to the Lochsa River to the Clearwater River which flowed into the Snake River, the Columbia River which discharged into the Pacific Ocean (<xref ref-type="fig" rid="fig2">Figure 2</xref>) and (<xref ref-type="fig" rid="fig1">Figure 1</xref>0). They were also among the first Europeans to view the Snake River Watershed (<xref ref-type="fig" rid="fig2">Figure 2</xref>).</p><p>After Lewis and Clark completed their mission and returned to Washington, DC, many more American explorers traveled to, and through, the Snake River and its major tributaries area between 1806 and 1811. Many of these explorers were on the original Lewis and Clark Expedition.</p><p>Later American fur trappers came looking for beaver streams. However, in 1819, the Hudson’s Bay Company sent Canadian fur trappers into the Snake River watershed to kill as many beavers as possible to discourage American trappers from coming to the Oregon territory. The territory included the future states of Oregon, Washington, Idaho and parts of Wyoming and Montana (<xref ref-type="fig" rid="fig1">Figure 1</xref>0). The Pacific Northwest area was eventually annexed by United States.</p></sec><sec id="s2_6"><title>2.6. Columbia River Gateway</title><sec id="s2_6_1"><title>2.6.1. Indigenous Peoples</title><p>Olson and Suski [<xref ref-type="bibr" rid="scirp.129687-ref12">12</xref>] found most historians believed “that Chinese or Japanese vessels, blown off course, reached the North American Coast long before the Europeans [<xref ref-type="bibr" rid="scirp.129687-ref13">13</xref>] . Archaeologists found evidence of human activity in the Columbia watershed near the confluence of the Snake (<xref ref-type="fig" rid="fig1">Figure 1</xref>0) and (<xref ref-type="fig" rid="fig1">Figure 1</xref>2) and Palouse rivers. About 3500 years ago humans transitioned to a more sedentary lifestyle based primarily on availability of salmon” [<xref ref-type="bibr" rid="scirp.129687-ref12">12</xref>] .</p><p>“Native Americans encountered foreigners in the 1700s and 1800s. In 1793, Alexander Mackenzie, the Canadian explorer, crossed the Columbia River valley in British Columbia. American and European vessels explored the coastal area around the mouth of the Columbia River in late 18<sup>th</sup> century and traded with the natives [<xref ref-type="bibr" rid="scirp.129687-ref13">13</xref>] . The contact proved deadly for Native American tribes as a large part of their population was lost after a smallpox epidemic” [<xref ref-type="bibr" rid="scirp.129687-ref12">12</xref>] .</p><p>Moulton [<xref ref-type="bibr" rid="scirp.129687-ref8">8</xref>] documented “from 1805 to 1806 the Lewis and Clark Expedition traveled through the Oregon territory along the Clearwater, Snake and Columbia rivers. They encountered small settlements of Native Americans. The Indigenous groups of the lower and mid-Columbia River valley were not tribal. The social units were smaller than a village and often at a family level. The units would shift with the season, following the salmon up and down the river and tributaries. Historically, the Columbia River salmon and steelhead runs averaged 10 to 16 million fish annually. The most productive and significant Native fishing site was located at Celilo Falls. The establishment of salmon canneries by European settlers in 1866 reduced the salmon population dramatically” [<xref ref-type="bibr" rid="scirp.129687-ref8">8</xref>] .</p></sec><sec id="s2_6_2"><title>2.6.2. Willamette Valley</title><p>Olson and Suski [<xref ref-type="bibr" rid="scirp.129687-ref12">12</xref>] reported “the 240 km Willamette Valley in Oregon was the favored destination of immigrants using the Oregon Trail, and was promoted in the 1820s as the “Land of Flowing Milk and Honey” (<xref ref-type="fig" rid="fig1">Figure 1</xref>3) and (<xref ref-type="fig" rid="fig1">Figure 1</xref>4). The Willamette Valley has a Mediterranean climate with oceanic features; summers are dry, cloudless and hot, while winters are consistently cloudy, cool and rainy [<xref ref-type="bibr" rid="scirp.129687-ref14">14</xref>] . The steel-covered wheels of the oxen-drawn wagon trains cut into exposed batches of bedrock, and are still visible 200 years later (<xref ref-type="fig" rid="fig1">Figure 1</xref>3).</p><p>“The Willamette River flows through the entire Willamette Valley and into the Columbia River north of the current city of Portland. It is surrounded on three sides by the Oregon Coast Range to the west, the Cascade Range to the east and the Calapooya Mountains to the south. The broad flat plain is Oregon’s most agriculturally productive area. Much of the Willamette Valley soil fertility is derived from a series of massive ice-age floods (<xref ref-type="fig" rid="fig1">Figure 1</xref>0) that came from Lake Missoula in Montana. These floods scoured the landscape across eastern Washington and carried sediment and soil down the tributaries to the Columbia River Gorge. Floodwaters backed up by ice jams flooded the valley to a depth of 90 to 120 m, and, if the floodwaters were at that depth today, modern-day Portland, with an elevation of 6.1 m above sea level, would be under water. The lake gradually drained leaving highly fertile alluvial soils and layered sedimentary soils on the Willamette Valley floor to a height of about 55 to 61 m above current sea level. The sediments, soils, alluvial and lacustrine parent materials are up to 1 km thick in the valley” [<xref ref-type="bibr" rid="scirp.129687-ref12">12</xref>] .</p></sec></sec><sec id="s2_7"><title>2.7. Oregon Trail the Platte River Gateway to the Pacific Northwest</title><p>Olson and Morton [<xref ref-type="bibr" rid="scirp.129687-ref7">7</xref>] determined “the Oregon Trail, 1820s to 1850s, is located adjacent to the Platte and North Platte rivers (<xref ref-type="fig" rid="fig5">Figure 5</xref>) and then proceeds overland to the Snake River. The wooden wheels had steel covered rims which cut grooves into any exposed bedrock (<xref ref-type="fig" rid="fig1">Figure 1</xref>3) and can still be seen 150 years later. Many American settlers traveled the Oregon Trail using horses and Conestoga wagons. The final destination was the Willamette Valley in Oregon (<xref ref-type="fig" rid="fig1">Figure 1</xref>0) south of Columbia River” [<xref ref-type="bibr" rid="scirp.129687-ref7">7</xref>] .</p><p>“The North and South Platte rivers emerge from the Rocky Mountain snowmelt in Colorado and Wyoming and meet near the town of North Platte, Nebraska, to form the Platte River (<xref ref-type="fig" rid="fig6">Figure 6</xref>). The Platte River, about 500 km long, is a major tributary of the lower Missouri River and has a 77,000 km<sup>2</sup> basin. The North Platte River watershed encompasses 80,000 km<sup>2</sup>, and the South Platte River drainage basin is 73,000 km<sup>2</sup>. The broad, shallow Platte River is not easily navigated due to its muddy silted bottom and numerous islands. Although it was not a good river to canoe and was never used as a major water route, it was an important trail marker for settlers and adventurers traveling west to Oregon, California, Utah, and Montana [<xref ref-type="bibr" rid="scirp.129687-ref9">9</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref10">10</xref>] [<xref ref-type="bibr" rid="scirp.129687-ref11">11</xref>] . Today many of the river valleys and bottomlands in this region are used for irrigated agriculture that is made possible by eight dams on the North Platte River and 20 on the South Platte River” [<xref ref-type="bibr" rid="scirp.129687-ref7">7</xref>] .</p><p>The Oregon Trail had been established by the middle of the 19<sup>th</sup> century, and generally followed the Platte, Snake, and Columbia rivers. However, settlers had to cross the Snake River [<xref ref-type="bibr" rid="scirp.129687-ref7">7</xref>] . One ferry crossing was near present day Glenn’s Ferry (<xref ref-type="fig" rid="fig1">Figure 1</xref>1). Another ferry crossing of the Snake River was created near the mouth of the Boise River called “Three Island Crossing” or “Two Island Crossing”. This area split the river into three or four 61 m wide channels. Emigrants crossed the Snake and proceeded down the west bank before re-crossing the river near Fort Boise. The Lower Snake River then passes through a 1.6 km deep gorge known as Hells Canyon (<xref ref-type="fig" rid="fig1">Figure 1</xref>5), the deepest river gorge in North America [<xref ref-type="bibr" rid="scirp.129687-ref8">8</xref>] . Overland settlers would continue down the drier east side of the gorge. Others floated their wagons on the Snake and Columbia rivers to the Willamette River and valley (<xref ref-type="fig" rid="fig3">Figure 3</xref>). The Willamette Valley, the destination of choice for the travelers on the Oregon Trail [<xref ref-type="bibr" rid="scirp.129687-ref1">1</xref>] was often referred to as the “Land of Flowing Milk and Honey” (<xref ref-type="fig" rid="fig1">Figure 1</xref>6).</p></sec></sec><sec id="s3"><title>3. Conclusion</title><p>The study documents the key role of rivers in the United States territorial expansion in the 19th century from the Mississippi River to the Pacific Ocean. In 1803, President Jefferson commissioned Meriwether Lewis and William Clark to explore the Missouri River watershed and find a water route to the Pacific Ocean. With such a river connection, President Jefferson hoped to claim additional North American lands between the continental divide in the Rocky Mountains and the Pacific Ocean. The Lewis and Clark expedition trip found no such waterway link; however, they did continue on to explore the lands west of the continental divide. The Pacific Northwest land was not included in the 1803 Louisiana Purchase (<xref ref-type="fig" rid="fig1">Figure 1</xref>) [<xref ref-type="bibr" rid="scirp.129687-ref14">14</xref>] . The only way that the Pacific Northwest could be claimed as part of the United States territory was for Americans to settle there before the Canadians. North West Company outposts were already established on the Columbia River. The previous Lewis and Clark Expedition successful trip was used by President Jefferson to encourage Americans to travel, via the Oregon Trail which paralleled the Platte and Snake rivers (<xref ref-type="fig" rid="fig2">Figure 2</xref>), to settle and establish the United States claim the Pacific Northwest. Starting in the 1820s, many Americans traveled via the Oregon Trail to the Willamette Valley in Oregon. American settlement in the Pacific Northwest allowed President Jefferson’s vision of the United States extending across North America from the Atlantic to the Pacific Ocean, to become a reality.</p></sec><sec id="s4"><title>Acknowledgements</title><p>A special thanks to Cory Suski, Sam Indorante, John Crivello, Lois Wright Morton, and Jerry Miller who assisted in the research, writing, distribution and archival of cited Great Rivers of North America publications.</p></sec><sec id="s5"><title>Conflicts of Interest</title><p>The author declares no conflicts of interest regarding the publication of this paper.</p></sec><sec id="s6"><title>Cite this paper</title><p>Olson, K.R. (2023) The Role of Rivers in the United States 19th Century Territorial Expansion from the Mississippi River to the Pacific Ocean. 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