Force Second Div To Collapse Beneath First Div When Browser Responds

I have two major <divs> on the index page of this site:

http://htmlmesomething.drawyourpets.com/

I'm trying to make it so that the second container / div (and its contents) collapses beneath the contents of the first div at tablet and mobile sizes.

HTML

<div class="container_one">

<section id="column_one">
    <h1>Bruce</h1>
    <img src="images/bruce.jpg"/>
    <p>Bruce was a red tailed black shark who died from accidental poisoning.
         We were given bad instructions from a store owner on how to properly use cleaning
          solution for his aquarium, and the rapid increase in PH levels caused Bruce to 
         suffocate within minutes.</p>
</section>

<section id="column_two">
    <h1>Captain</h1>
    <img src="images/captain.jpg"/>
    <p>Captain was a yellow lab who died from congestive heart failure. He 
        had been on medication for his condition, and it had likely kept him alive
         for years, until eventually he became too obese for the medication to take effect,
        and he died suddenly.
         </p>
</section>

<section id="column_three">
    <h1>Tigger</h1>
    <img src="images/tigger.jpg"/>
    <p>Tigger was a red Tabby cat who struggled with obesity. He died on Christmas
        when a blood clot to the brain caused him to collapse and have a siezure.</p>
</section>

</div><!--ends first container-->

<div class="container_two">

<section id="solo_column">
    <img id="slip" onmouseover="this.src='images/slip2.jpg'" onmouseout="this.src='images/slip1.jpg'" height="100px" width="100px" src="images/slip1.jpg">
</section>

</div><!--ends second container-->

CSS (DEKSTOP)

.container_one {
padding: 5px;
width: 960px;
height:340px;
margin: 20px auto;
}

.container_two {
padding: 5px;
width: 960px;
height:340px;
margin: 20px auto;
}

#column_one {
width: 290px;
float: left;
padding: 20px 15px;
text-align:center;
background-color:#E8E6E6;
border: solid 1px grey;
}

#column_two {
width: 294px; /*Account for margins, border values*/
float: left;
padding: 20px 15px; 
margin: 0px 5px 5px 5px;
text-align:center;
background-color:#E8E6E6;
border: solid 1px grey;
}

#column_three {
width: 270px;
float: left;
padding: 20px 15px;
text-align:center;
background-color:#E8E6E6;
border: solid 1px grey;
}

#solo_column {
width: 100%;
float: none;
padding: 20px 15px;
text-align:center;
}

.footer_info {
text-align:center;
margin-top:35px;
}

CSS (TABLET):

/*980px or less*/
@media screen and (max-width: 980px) {

div, section {
    display:block;
    height:320px
}

.container_one {
    width: 94%;
}

.container_two {
    width: 94%;
    clear:both;
    float: right;
}

#column_one {
    width: 41%;
    padding: 1% 4%;
    height: 355px;
}

#column_two {
    width: 41%;
    padding: 1% 4%;
    margin: 0px 0px 12px 5px;
    float: right;
    height: 355px;
}

#column_three {
    clear: both;
    padding: 1% 4%;
    width: auto;
    float: none;
}

header, footer {
    padding: 1% 4%;
}
article {
    margin-top:50px;
}   
footer, .footer_info {
    clear:both;
    margin-top:205px;
}
}

CSS (TABLET)

/* for 600px or less */
@media screen and (max-width: 600px) {

#column_one {
    width: auto;
    float: none;
    margin-bottom: 12px;
}

#column_two {
    width: auto;
    float: none;
    margin-left: 0px;
}

#column_three {
    width: auto;
    float: none;
}

#solo_column {
    width: auto;
    float: none;
}

footer, .footer_info {
    clear:both;
    margin-top:405px!important;
}

.container_two {
    clear:both;
    float: right;
}

}

CSS (MOBILE)

/*for 480px or less*/
@media screen and (max-width: 480px) {

#column_one {
    height: 385px;
}
#column_two {
    height: 385px;
}
#column_three {
    height: 385px;
}
#solo_column {
    height: 385px;
}
h1 {
    font-size: 2em;
}
footer .footer_info {
    clear:both;
    margin-top:500px!important;
}

.container_two {
    clear:both;
    float: right;
}

}

I tried adding clear:both to the container_two class.

I didn't add this rule to the class at desktop size because it's already appearing below the other <div>.

How I can get this second <div> to clear or collapse beneath the first <div> at tablet and mobile sizes?

EDIT: Tried adding float: right to the .container_two div classes.

EDIT 2: Also tried adding <div style="clear: both;"></div> to the HTML after the first <div>.