multiple drop down <select> menus with only one selectable at a time

Question

I have 4 <select> drop down menus with corresponding values that interact with the iframe on my website. The issue is that I want to only allow one drop down to be selectable at a time and for the other drop down menus to return to the original value when the website first opened. After researching for solutions I was not able to find any online. Below I will include the code for the drop down menus.

 <script>
    var gST = ""

    function set_st(t) {
       var sel_el = t.options[t.selectedIndex]
       gST = sel_el.value
        document.getElementById('my-iframe').src = 'http://watermonitor.gov/naww/index.php?st=' + gST
    }

    function down_ch() {
        var link = "http://waterwatch.usgs.gov/download/?gt=map&st="+gST+"&ofmt=kmr&x=0";   
        window.open(link)
    }

    function setURL(url){
    document.getElementById('my-iframe').src = url;
    }

  </script>

<header>
    <div id='map_container' style="margin-top: 100px; border: 0px solid red; text-align: center;">
      <div id="xbuilder">
        <table>
          <tbody>
            <tr>
              <td>
                <div>
                  United States: 
                  <span data-tooltip='Choose a US state'>
                    <select name='usst' id='usst' size='1' onchange='javascript:set_st(this);' >
                      <option value=''>States</option>
                      <option value='al'>Alabama</option>
                      <option value='ak'>Alaska</option>
                      <option value='az'>Arizona</option>
                      <option value='ar'>Arkansas</option>
                      <option value='ca'>California</option>
                      <option value='co'>Colorado</option>
                      <option value='ct'>Connecticut</option>
                      <option value='de'>Delaware</option>
                      <option value='dc'>District of Columbia</option>
                      <option value='fl'>Florida</option>
                      <option value='ga'>Georgia</option>
                      <option value='hi'>Hawaii</option>
                      <option value='id'>Idaho</option>
                      <option value='il'>Illinois</option>
                      <option value='in'>Indiana</option>
                      <option value='ia'>Iowa</option>
                      <option value='ks'>Kansas</option>
                      <option value='ky'>Kentucky</option>
                      <option value='la'>Louisiana</option>
                      <option value='me'>Maine</option>
                      <option value='md'>Maryland</option>
                      <option value='ma'>Massachusetts</option>
                      <option value='mi'>Michigan</option>
                      <option value='mn'>Minnesota</option>
                      <option value='ms'>Mississippi</option>
                      <option value='mo'>Missouri</option>
                      <option value='mt'>Montana</option>
                      <option value='ne'>Nebraska</option>
                      <option value='nv'>Nevada</option>
                      <option value='nh'>New Hampshire</option>
                      <option value='nj'>New Jersey</option>
                      <option value='nm'>New Mexico</option>
                      <option value='ny'>New York</option>
                      <option value='nc'>North Carolina</option>
                      <option value='nd'>North Dakota</option>
                      <option value='oh'>Ohio</option>
                      <option value='ok'>Oklahoma</option>
                      <option value='or'>Oregon</option>
                      <option value='pa'>Pennsylvania</option>
                      <option value='ri'>Rhode Island</option>
                      <option value='sc'>South Carolina</option>
                      <option value='sd'>South Dakota</option>
                      <option value='tn'>Tennessee</option>
                      <option value='tx'>Texas</option>
                      <option value='ut'>Utah</option>
                      <option value='vt'>Vermont</option>
                      <option value='va'>Virginia</option>
                      <option value='wa'>Washington</option>
                      <option value='wv'>West Virginia</option>
                      <option value='wi'>Wisconsin</option>
                      <option value='wy'>Wyoming</option>
                      <option value='pr'>Puerto Rico</option>
                    </select>
                  </span>
                  <span data-tooltip='Choose a US water resource region'>
                    <select name='ushuc' id='ushuc' size='1' onchange='javascript:set_st(this);' >
                      <option value=''>Water Res. Region</option>
                      <option value='01'>01 New England</option>
                      <option value='02'>02 Mid Atlantic</option>
                      <option value='03'>03 South Atlantic-Gulf</option>
                      <option value='04'>04 Great Lakes</option>
                      <option value='05'>05 Ohio</option>
                      <option value='06'>06 Tennessee</option>
                      <option value='07'>07 Upper Mississippi</option>
                      <option value='08'>08 Lower Mississippi</option>
                      <option value='09'>09 Souris-Red-Rainy</option>
                      <option value='10'>10 Missouri</option>
                      <option value='11'>11 Arkansas-White-Red</option>
                      <option value='12'>12 Texas-Gulf</option>
                      <option value='13'>13 Rio Grande</option>
                      <option value='14'>14 Upper Colorado</option>
                      <option value='15'>15 Lower Colorado</option>
                      <option value='16'>16 Great Basin</option>
                      <option value='17'>17 Pacific Northwest</option>
                      <option value='18'>18 California</option>
                      <option value='19'>19 Alaska</option>
                      <option value='20'>20 Hawaii</option>
                      <option value='21'>21 Puerto Rico</option>
                    </select>
                  </span>
                </div>
                    <button class="button" onClick="javascript:down_ch();"><span class="icon">Download Data</span></button>
              </td>
            </tr>
            <tr>
              <td>
                <div>
                  Canada:
                  <span data-tooltip='Choose a Canadian province'>
                    <select name='nus' id='nus' size='1' onchange='javascript:set_st(this);' >
                      <option value=''>Provinces</option>
                      <option value='zab'>Alberta</option>
                      <option value='zbc'>British Columbia</option>
                      <option value='zmb'>Manitoba</option>
                      <option value='znb'>New Brunswick</option>
                      <option value='znl'>Newfoundland and Labrador</option>
                      <option value='zns'>Nova Scotia</option>
                      <option value='znt'>Northwest Territories</option>
                      <option value='znu'>Nunavut</option>
                      <option value='zon'>Ontario</option>
                      <option value='zpe'>Prince Edward Island</option>
                      <option value='zqc'>Quebec</option>
                      <option value='zsk'>Saskatchewan</option>
                      <option value='zyt'>Yukon</option>
                    </select>
                  </span>
                  <span data-tooltip='Choose a Canadian river basin'>
                    <select name='nhuc' id='nhuc' size='1' onchange='set_st(this);' >
                      <option value=''>Canadian River Basins</option>
                      <option value='z01'>01 Maritime Provinces Drainage</option>
                      <option value='z02'>02 St.Lawrence River Drainage</option>
                      <option value='z03'>03 Northern Quebec Drainage</option>
                      <option value='z04'>04 Southwest Hudson Bay Drainage</option>
                      <option value='z05'>05 Nelson River Drainage</option>
                      <option value='z06'>06 Western Hudson Bay Drainage</option>
                      <option value='z07'>07 Great Slave Lake Drainage</option>
                      <option value='z08'>08 Pacific Drainage</option>
                      <option value='z09'>09 Yukon River Drainage</option>
                      <option value='z10'>10 Arctic Drainage</option>
                      <option value='z11'>11 Mississippi River Drainage</option>
                    </select>
                  </span>
                </div>
                <div style='margin-left: 10px; float: right;' data-tooltip="Click to view a map of specified region."><input  value="Reset Map" type="button" onclick="setURL('http://watermonitor.gov/naww/index.php')"/></div>
              </td>
            </tr>
          </tbody>
        </table>
      </div>
    </div>
  </header>

Answer 1

You can probably achieve this with events. Eg when the first drop down list is opened, then set all of the others to disabled: http://html.com/attributes/select-disabled/

You could do that with jquery like this:

$(ele).attr(disabled, disabled);

For the events, check out this: http://api.jquery.com/category/events/

look up the change event, once the event fires you'll need to set the other drop down lists to the original value eg:

$(ele).val('0');

You may want to update your question with what should happen when the page loads, and then what should happen when you open drop down lists on the page.